New Zealand's stormwater and wastewater systems are particularly vulnerable to climate change, says a report citing Edgecumbe's experience as an example of the challenges.

In April, the Bay of Plenty town was swamped by flooding, leading to raw sewage floating through the streets and making the clean-up difficult.

Six months on, 500 houses remain unliveable and flood-proofing the town remains a distant goal, the report commissioned by the NIWA-led Deep South National Science Challenge says.

The country's stormwater and wastewater assets are valued at well over $20 billion.
However, the report says, much of it was not designed for climate change.


Adapting to the "increasingly severe risks" of extreme rainfall, storm surges, sea level rise and drought could require significant and expensive changes to networks.

"For example, in many local water systems, roads are designed to be used as a secondary stormwater routes in extreme flooding," co-author Professor Iain White said.

"This is fine in most situations but, in extreme inundation events, wastewater containing sewage may mix with the stormwater overflows, which of course brings problems such as we saw in Edgecumbe."

Prof White, from Waikato University, said the increase in extreme rainfall events would also add stress by overwhelming the networks.

He said drought would bring its own problems, disrupting gravity systems by slowing flow and leading to blocked pipes.

Particularly lengthy droughts could also affect wastewater treatment processes, creating functional and safety concerns.

Prof White said a priority for Deep South Challenge was further research to better understand the risks.

Once that knowledge was available, it would be a case of considering the most appropriate response to help reduce those impacts.