Climate change: Where in New Zealand could be worst hit?

Sewage contamination dogged the clean-up of flood-hit Edgecumbe last year. A new study seeks to better understand climate change's threat to sewer systems. Photo / File

A future of extreme weather and higher seas could overwhelm New Zealand's $20b sewer network - and now researchers have set out to reveal which areas could be most at risk.

A new year-long investigation will take a closer look at the country's stormwater and wastewater systems, after an initial study found much of the infrastructure could be inundated by a threat it was never designed for.

Heaping pressure on some 24,000km of public wastewater networks, more than 3000 pump stations, and 17,000km of stormwater channels could be sea levels, which are projected to rise up to a metre by the end of the century, and the likelihood of more frequent extreme rainfall events.

Many local water systems have been designed only to be used as secondary stormwater routes in extreme flooding.

But in the kind of weather that could come with climate change, sewage could gush into stormwater overflows, causing contamination problems like those which dogged the clean-up of flooded Edgecumbe last year.

In coastal areas, especially, rising oceans and more frequent big storms could lead to sewage overflows, damaged and corroded pipes, liquefaction, and electrical failure at treatment plants.

"Aside from the obvious impacts we are aware of, and those we are beginning to understand, there is so much that we actually don't know," the project's leader, Tonkin and Taylor climate and resilience specialist James Hughes, said.

"For someone working in this field this can be both very worrying and intriguing at the same time."

The new study, supported by the Deep South National Science Challenge, will trawl through research to pin-point the areas in the firing line.

That would mean assessing where impacts might be more likely to occur, or hit hardest, because of factors like ageing networks, vulnerability to flash-flooding, pressure from development, or rising groundwater.

"For example, sea level rise over time will affect groundwater levels, which in turn may reduce the ability of septic tanks to drain via dispersal fields," Hughes said.

"This then may have environmental implications, as well as financial, social and cultural implications for landowners and communities."

The other big question was simply what range of effects - and flow-on effects - climate change might bring.

A panel of experts from universities, government, councils, iwi and water agencies is supporting the work.

Hughes said the study would ultimately offer councils and communities a clearer picture of the threats, including where the trouble spots were and how big the impacts might be.

"We won't be looking at specific solutions. However, by clearly understanding the impacts and implications on stormwater and wastewater infrastructure, solutions can be developed that will have a much greater chance of success."

The investigation comes after a Government-commissioned working group found New Zealand lacked a co-ordinated plan to deal with future climate change, despite its risk to hundreds of billions of dollars of property and infrastructure.

It revealed a range of weak spots and knowledge gaps - notably that there was no nationally co-ordinated strategy, and that New Zealand tended to react to events instead of preparing for them.

It found inconsistencies in timeframes across legislation affecting adaptation, and some "competing objectives" across legislation and policies, meaning roles could be confused and investing in resources was made challenging.

The Government has responded by releasing new guidance setting out a 10-step process for councils and communities, based on the latest science.

Further work on adaptation is under way, and another working group report on how New Zealand can adapt to climate change is due in March.