What are the Legionella testing thresholds for cooling towers in NSW?

Under AS/NZS 3666.3 and the Public Health Regulation 2022, Legionella at or above 10 CFU/mL requires urgent control measures. At or above 1,000 CFU/mL, you must notify local government within 24 hours via Approved Form 4. Monthly testing using NATA-accredited laboratories is mandatory.

How often must cooling towers be tested for Legionella in Australia?

The Public Health Regulation 2022 (NSW) requires monthly sampling and testing for both Legionella and heterotrophic colony count (HCC). All testing must use NATA-accredited laboratories. Results must be documented on Approved Form 3.

What is required to register a cooling tower in NSW?

Occupiers must notify local government within 1 month of installation using Approved Form 6 and pay a $120 fee. The council assigns a unique identification number that must be displayed on each tower within 30 days. A risk management plan must be prepared every 5 years and independently audited every 12 months.

What drift eliminator performance does AS/NZS 3666.1 require?

AS/NZS 3666.1:2011 requires drift eliminators to limit drift loss to no more than 0.002% of the circulating water flow rate. This applies to both new installations and retrofits of existing towers.

What are the penalties for cooling tower non-compliance in NSW?

Under the Public Health Act 2010 and Public Health Regulation 2022, penalties include $220 per individual or $440 per corporation for offences such as failure to notify installation, failure to display ID numbers, or failure to have documents available. Failure to comply with a Prohibition Order carries fines of $1,650 per individual or $3,300 per corporation.

Cooling Tower Water Treatment and Compliance

1. What You Need to Know

Cooling towers reject heat by evaporating water. That evaporation concentrates minerals, creates warm moist conditions, and produces aerosol drift. This combination breeds bacteria - most critically, Legionella pneumophila. A single contaminated tower can spread Legionnaires' disease across an entire precinct.

The Public Health Act 2010 (NSW) and AS/NZS 3666 make cooling tower water treatment a legal obligation, not a maintenance choice. Every tower needs registration, monthly testing, a documented risk management plan, and annual independent audits.

Legionella grows between 20°C and 45°C. Cooling towers operate right in that range. Without active chemical treatment and regular monitoring, bacterial counts can spike from safe levels to dangerous concentrations within days.

The stakes are real. Legionella counts above 1,000 CFU/mL trigger mandatory 24-hour notification to local government. Outbreak investigations shut down buildings. Fines apply for non-compliance, and prosecution is possible under the Public Health Act.

2. The Rules

AS/NZS 3666.1:2011 (Design, Installation and Commissioning)

Design drift eliminators to limit drift loss to 0.002% of circulating water flow rate
Provide safe and easy access for cleaning, inspection, and maintenance
Locate air intakes away from cooling tower discharge
Install systems to allow complete drainage and cleaning

AS/NZS 3666.2:2011 (Operation and Maintenance)

Maintain water treatment programs for biocide dosing, corrosion inhibition, and scale control
Keep maintenance records on site for a minimum of 12 months
Clean systems at the frequency specified in the risk management plan

AS/NZS 3666.3:2011 (Performance-Based Maintenance)

Test for Legionella and heterotrophic colony count (HCC) every month
Legionella target: not detected or below 10 CFU/mL
HCC target: below 100,000 CFU/mL
Use NATA-accredited laboratories for all microbial testing

Public Health Regulation 2022 (NSW)

Register cooling towers with local government within 1 month of installation (Approved Form 6, $120 fee)
Display the assigned unique identification number on each tower within 30 days
Prepare a risk management plan every 5 years (or more frequently for high-risk systems)
Submit the RMP certificate to local government within 7 days
Complete an independent audit every 12 months with no gaps between audit periods
Submit audit completion certificates to local government within 7 days
Notify local government within 7 days of decommissioning

Reportable Test Results (notify within 24 hours via Approved Form 4)

Legionella: 1,000 CFU/mL or above
HCC: 5,000,000 CFU/mL or above

Penalty Infringements (NSW)

Failure to notify installation: $220 individual / $440 corporation
Failure to display identification number: $220 / $440
Failure to have documents available for inspection: $220 / $440
Failure to comply with a Prohibition Order: $1,650 / $3,300

3. What This Means in Practice

Legionella grows fast when treatment lapses. A cooling tower that loses biocide dosing for even a few days can see bacterial counts jump from below detection to thousands of CFU/mL. Automated dosing systems with alarm monitoring eliminate the gap between biocide depletion and replenishment. Manual dosing programs need daily checks at minimum.

Concentration cycles drive water quality. As water evaporates, dissolved solids concentrate. Run too many cycles and scale forms on heat exchange surfaces. Run too few and you waste water and chemicals. Most towers operate best between 3 and 6 cycles of concentration, depending on makeup water quality. Conductivity controllers automate blowdown to maintain the target range.

Biofilm is harder to kill than free-floating bacteria. Once Legionella colonises biofilm on tower surfaces, standard biocide doses cannot reach it. The bacteria shelter inside the biofilm matrix. Preventing biofilm formation through consistent oxidising biocide programs matters more than trying to remove it after the fact. If biofilm establishes, you need a full decontamination - drain, clean, hyperchlorinate, and refill.

Drift eliminators are your frontline public health control. They capture water droplets before they leave the tower. AS/NZS 3666.1 limits drift to 0.002% of circulating flow. Damaged or missing drift eliminators turn a contaminated tower into an aerosol generator that can spread Legionella hundreds of metres downwind.

Monthly testing catches problems before they become outbreaks. The 24-hour reporting threshold for Legionella sits at 1,000 CFU/mL. But action starts much earlier - at 10 CFU/mL or above, you must implement urgent control measures. The gap between 10 and 1,000 is your window to fix the problem before it triggers regulatory notification.

Records must survive inspection. Council inspectors can request cooling tower maintenance records with just 4 hours' notice. Keep all Approved Form 3 monthly reports, test results, chemical delivery dockets, and cleaning records in a single accessible location - either a physical binder at the tower or a digital system that staff can access on site.

4. Key Design Decisions

Chemical treatment program type

Choose between oxidising biocides (chlorine, bromine), non-oxidising biocides (isothiazolones, glutaraldehyde), or a combination program. Oxidising biocides work fast and break down biofilm but degrade in sunlight and high-pH water. Non-oxidising biocides penetrate biofilm better but take longer to act. Most commercial towers run a combination: continuous low-dose oxidising biocide with periodic non-oxidising shock treatment.

Tradeoff: Combination programs cost more in chemicals and dosing equipment. Single-chemistry programs are cheaper but less effective against established biofilm. The combination approach gives you redundancy - if one chemistry fails, the other provides backup.

Automated vs manual dosing

Automated dosing systems use conductivity and ORP (oxidation-reduction potential) sensors to control blowdown and biocide injection. Manual programs rely on service technicians visiting weekly or fortnightly. Automated systems respond to load changes in real time. Manual programs always lag behind conditions.

Tradeoff: Automated dosing systems add $15,000–$30,000 to capital cost. Manual programs cost less upfront but carry higher risk of treatment gaps. For towers serving occupied buildings, automated dosing is the standard of care. Manual programs suit small or seasonal systems where the risk profile is lower.

Makeup water pre-treatment

Poor makeup water creates treatment problems. High hardness causes scale. High iron feeds bacteria. High organic content consumes biocide. Consider side-stream filtration, water softening, or reverse osmosis depending on the source water quality. Test makeup water before selecting the treatment program.

Tradeoff: Pre-treatment adds capital and operating cost. Without it, the tower treatment program works harder and costs more in chemicals. Pre-treatment pays for itself on towers running more than 4 cycles of concentration with hard water above 200 mg/L as CaCO₃.

Access and maintenance provisions

AS/NZS 3666.1 requires safe and easy access for cleaning, inspection, and maintenance. This means platforms with handrails around the tower, access doors large enough for a person to enter the basin, adequate lighting, and isolation valves on all chemical feed lines. Design these in from the start. Retrofitting access to an operational tower costs three times as much and disrupts building operations.

Tradeoff: Generous access provisions increase the structural footprint and cost. Tight access saves space but makes maintenance difficult, slow, and expensive. Technicians who cannot reach equipment safely will skip tasks. That leads to treatment failures.

5. Callouts

For Contractors

Check drift eliminators during every service visit. Missing or damaged sections compromise public health protection. Replace damaged drift eliminator packs immediately - do not wait for the next scheduled maintenance window.

Verify biocide dosing pump operation monthly. Check calibration, confirm chemical supply levels, and test the ORP or residual reading against a portable meter. A dosing pump that runs dry or loses prime leaves the tower unprotected.

For Developers

Budget for ongoing water treatment costs from day one. A typical commercial cooling tower costs $8,000 to $15,000 per year in water treatment chemicals, monthly testing, and annual audits. This is not optional - it is a regulatory requirement under the Public Health Act 2010.

Register every cooling tower with local government before commissioning. The $120 notification fee is trivial compared to the penalties for operating an unregistered system. Build the registration timeline into your project program alongside the occupancy certificate.

For Architects

Locate cooling towers away from fresh air intakes, occupied terraces, and pedestrian areas. Drift can carry Legionella-contaminated aerosol. AS/NZS 3666.1 and the NCC both require separation between cooling tower discharge and outdoor air intakes.

Allow space around the tower for maintenance access. Water treatment technicians need room to carry chemical drums, access dosing equipment, and enter the tower basin for cleaning. A 1,500 mm clear zone around all sides of the tower is the practical minimum.

6. References

AS/NZS 3666.1:2011, Air-handling and water systems of buildings – Microbial control – Part 1: Design, installation and commissioning
AS/NZS 3666.2:2011, Air-handling and water systems of buildings – Microbial control – Part 2: Operation and maintenance
AS/NZS 3666.3:2011, Air-handling and water systems of buildings – Microbial control – Part 3: Performance-based maintenance of cooling water systems
Public Health Act 2010 (NSW)
Public Health Regulation 2022 (NSW)
NSW Health Fact Sheet: Legionella control in cooling water systems – regulatory requirements at a glance
NSW Health Fact Sheet: Managing cooling water systems throughout their lifecycle
enHealth Guidelines: Guidelines for Legionella control (2015)