Is Your Rainwater Tank Making Your Family Sick?

New research has revealed a disturbing truth about rainwater tanks across New Zealand: two-thirds of tanks tested contained dangerous levels of E. coli, putting hundreds of thousands of rural households at risk. If your home, farm, or lifestyle block relies on a rainwater tank for drinking water, this is essential reading.

The Hidden Danger in Your Rainwater Tank

Most New Zealanders assume rainwater is clean. After all, it falls straight from the sky. But a landmark 2025 University of Auckland study — Water Quality of Domestic Self-Supply in New Zealand (Joshi & Mankelow, 2025) — found the reality is far more alarming.

Researchers analysed hundreds of domestic rainwater tanks across the North Island and found:

• 67% of tanks exceeded acceptable levels of E. coli under New Zealand Drinking Water Standards
• 86% of tanks contained Enterococci — another indicator of faecal contamination
• 58% of tanks failed the recommended pH standards
• 100% of samples showed elevated water hardness
• In Taranaki, over 60% of tanks exceeded ammonia guidelines

More than 15% of New Zealand's population relies on domestic self-supplied systems like rainwater tanks. Unlike town water supplies, these systems are typically unmonitored and untreated, and there is no regulatory requirement for regular testing of private water supplies. According to the Ministry of Health, 35,000 New Zealanders are registered sick every year from water-borne diseases.

"The fact that most tanks tested exceeded acceptable E. coli levels is a clear reminder that untreated rainwater is not always as clean as people assume." — Dan Hawke, Co-Founder, Forsana

How Does Rainwater Become Contaminated?

When rain falls on your roof, it picks up everything that has accumulated there and then washes it directly into your storage tank. The main contamination sources are:

• Bird droppings — a major source of E. coli, Salmonella, and Campylobacter
• Rodent faeces — rats and mice near roofs and gutters are extremely common in rural areas
• Organic debris — decomposing leaves, moss, and lichen create ideal conditions for bacterial growth
• Dust and airborne pollutants — agricultural spray drift, vehicle emissions, and airborne particles accumulate on roof surfaces between rain events

Without treatment, contaminants sit in warm, dark conditions and multiply — particularly during summer months. The risk is highest after a dry spell followed by heavy rain, when a large volume of accumulated contaminants is suddenly washed into the tank.

Why Standard Rainwater Tank Treatments Fall Short

The New Zealand market offers several common approaches to rainwater tank treatment. Understanding their limitations helps explain why so many households are still at risk despite using off-the-shelf products.

Chlorine tablets (e.g. Farm Chlor)

Active ingredient: Sodium hypochlorite

Limitations: Produces harmful disinfection by-products (trihalomethanes) when reacting with organic matter. Strong chemical taste and odour. Degrades quickly in heat and sunlight.

Chlorine dioxide (e.g. DX50)

Active ingredient: Chlorine dioxide

Limitations: Requires precise two-part activation and measurement. Still introduces chlorine-based compounds. Ongoing chemical residue concerns for drinking water.

Ozone filtration systems (e.g. Tankman)

Active ingredient: Ozone (O₃)

Limitations: Requires professional installation and ongoing maintenance. High upfront cost ($500–$2,000+). Not practical for most rural households.

Standard hydrogen peroxide

Active ingredient: H₂O₂

Limitations: Effective antimicrobial — but standard formulations degrade rapidly when exposed to light, heat, and organic matter, losing potency before doing the job properly.

Forsana Tank Water Treatment

Active ingredient: Stabilised H₂O₂ + ionised silver

Why it’s different: No toxic by-products — breaks down to water and oxygen only. No chemical taste or odour. NZ hospital-grade disinfectant approval. Extended shelf life. Easy to use with no mixing required.

What's really getting into your water tank - Dan Hawke Listen Now

Why Forsana Tank Water Treatment Is Different

Forsana Tank Water Treatment uses a stabilised hydrogen peroxide formulation enhanced with ionised silver — a significant advancement over both standard hydrogen peroxide and conventional chemical treatments.

Hydrogen peroxide (H₂O₂) has been used as a disinfectant for over two centuries. Its key advantage is simple chemistry: when it breaks down, it produces only water and oxygen — leaving absolutely no toxic residue in your water supply or the environment.

Standard hydrogen peroxide degrades quickly when exposed to light, heat, or organic matter. Forsana's stabilised formulation addresses this directly. The addition of ionised silver significantly slows the degradation process, ensuring the disinfectant remains active for longer and delivers consistent, reliable protection.

What Makes Forsana Tank Water Treatment Superior

• No toxic by-products — breaks down to water and oxygen only, unlike chlorine which forms harmful trihalomethanes
• No chemical taste or odour in treated water
• Approved hospital-grade disinfectant in New Zealand — used in healthcare, commercial plant care, and vehicle rental sectors
• Stabilised formula — stays active far longer than standard hydrogen peroxide or chlorine tablets
• Eco-friendly — safe for gardens, septic systems, livestock, and waterways
• Simple to use — no mixing, no activation, no complicated two-part formulas
• Effective against E. coli, Enterococci, Salmonella, and a broad spectrum of bacteria and viruses

How to Treat and Maintain Your Rainwater Tank

Treatment is only part of the solution. A comprehensive tank water safety programme combines regular disinfection with good maintenance practices.

Annual Maintenance Checklist

1. Inspect your roof and gutters — clear debris, bird nests, and organic matter at least twice a year (spring and autumn)
2. Check and clean first-flush diverters — these divert the initial, most contaminated rainfall away from your tank
3. Inspect tank lid and screens — ensure entry points are sealed against rodents, insects, and light
4. Check overflow pipes — these should have fine mesh screens to prevent animal entry
5. Treat with Forsana Tank Water Treatment — follow the recommended dosing guidelines for your tank volume
6. Test your water annually — particularly if you have young children, elderly family members, or immunocompromised individuals in your household

When to Shock-Dose Your Tank

A shock dose treatment with Forsana Tank Water Treatment is recommended:

• After a dry spell followed by heavy rainfall — the highest contamination risk window
• If tank water develops an unusual taste, odour, or discolouration
• After any physical work on the tank or roof catchment system
• After flood events or if wildlife has gained access to the tank
• At least once per year as a preventative measure

Protecting Rural Families: The Bigger Picture

The University of Auckland research is a wake-up call for the estimated 750,000+ New Zealanders who rely on private water supplies. Unlike town water users, rural households and lifestyle block owners carry the full responsibility for the safety of their own water — with no regulatory requirement to test it.

Effective treatment is now more accessible than ever. Forsana Tank Water Treatment is available at over 105 retail outlets across New Zealand, primarily through Farmlands and Mitre 10 stores nationwide. For farmers and lifestyle property owners, managing tank water quality is just as important as maintaining any other part of your property's infrastructure.

Frequently Asked Questions

Is rainwater safe to drink without treatment?

Based on the latest University of Auckland research, untreated rainwater from domestic tanks carries significant health risks. Two-thirds of tanks tested exceeded acceptable E. coli levels under NZ Drinking Water Standards, and the Ministry of Health records 35,000 New Zealanders falling sick from water-borne diseases every year. Treatment with Forsana Tank Water Treatment is strongly recommended for any tank used as a drinking water source.

How often should I treat my rainwater tank?

At minimum, treat your tank once per year as a preventative measure. If your tank supplies drinking water to vulnerable household members, or if you notice any change in water quality, treat more frequently. After heavy rain following a dry period, a shock dose is strongly advisable.

Is hydrogen peroxide safe for drinking water?

Yes — when used at appropriate concentrations, hydrogen peroxide is safe and effective for water treatment. Forsana's stabilised formulation is an approved hospital-grade disinfectant in New Zealand. It breaks down into water and oxygen, unlike chlorine-based products which can leave harmful chemical by-products in your water supply.

Where can I buy Forsana Tank Water Treatment?

Forsana Tank Water Treatment is available at over 105 locations across New Zealand, primarily through Farmlands and Mitre 10 stores, and online at forsana.com.