Some experts believe the key to eliminating raw sewage overflows into Auckland Harbour lies on the roof of one of the city's recent office blocks.

A garden of low desert-like succulents and iceplants covers much of the roof of the Green Star-certified NZ Insurance building, built in Fanshawe St in 2009 and now housing the national office of the IAG insurance group.

IAG property manager Tim Griffith says the garden sits on a 50 to 75mm substrate of pumice and other inorganic and organic material. Rainwater soaks through it into a 38,000-litre tank which is used to flush the building's toilets.

The system reduces rainwater runoff to the city's drains by 75 per cent - and the benefits are not just environmental.


"There is also a financial benefit because you are not paying water charges," Griffith says.

Auckland University wastewater engineer Dr Lokesh Padhye says Auckland lags behind leading world cities by having no incentives for roof gardens, permeable paving or water tanks to reduce stormwater runoff into ageing combined stormwater/sewer systems.

"Many developed countries are facing these issues, but there have been a lot of mitigating measures taken for the last two or three decades," he says.

"The frequency of overflows [in Auckland] is a little bit of a concern, especially considering that Auckland wants to be the most liveable city in the world."

Green Building Council chief executive Andrew Eagles says Auckland's rainfall pattern is "possibly the best in the world for being dealt with by green roofs".

"The events are frequent medium rainfall events. This means green roofs are very well able to deal with them," he says.

He says Tokyo, Toronto and Copenhagen all require green roofs on new buildings above specified sizes or with flat roofs.

Many US cities including New York and Washington offer rates rebates, grants and other incentives for green roofs.


But Auckland Council Healthy Waters manager Craig Mcilroy opposes mandating such measures.

"The unitary plan provides and encourages green solutions. However they are only one tool used to mitigate the effects of urbanisation," he says.

"Rainwater re-use tanks are a useful way of reducing high flows in the drainage network, however they are not a replacement for a water supply system in an urban environment. When it's dry and you have to fill up the tank, the water has to come from a regional network."