Much of the country's wastewater and stormwater infrastructure is unprepared for the strain of climate change, which could lead to more floods like the one that devastated Edgecumbe unless billions are spent.

This is the finding of a report from the Deep South National Science Challenge, released on Friday, which warned that if action was not taken, communities could expect more water outages, contamination and sewage overflows.

Environmental planner Iain White, who co-authored the Climate Change and Stormwater and Wastewater Systems report, said aging infrastructure was expected to face more extreme rainfall events, storm surges, sea level rise and drought as the climate warms.

CHRIS McKEEN/STUFF Receding floodwaters reveal the damage to Edgecumbe after a flood bank was breached in April.

The Edgecumbe floods in April were a wake-up call for how unprepared some systems were, White said.

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MALCOLM GROWDEN/FACEBOOK Aerial footage over Edgecumbe shows the extent of flooding in the region.

After heavy rain brought about by Cyclone Debbie, the stopbank protecting Edgecumbe from the Rangitāiki River burst, flooding the town and leaving raw sewage floating in the streets.

Six months on, 500 houses were still unliveable and flood-proofing remained a distant goal, White said.

Existing infrastructure had been designed with last century in mind, and without upgrades Kiwis should expect failures more frequently, he said.

SUPPLIED Environmental planning professor Iain White says New Zealand's waste and storm water infrastructure may struggle as climate change takes effect.

"We are going to have to invest in bigger pipes, which are more expensive ... we are going to have to change how we fund our infrastructure," White said.

"If we want things to stay the same, we will have to change."

Most of the country's stormwater infrastructure was built to handle a one-in-10-year flood. But White said by 2050, it was expected some coastal areas could expect 1-in-100-year events on an almost annual basis.

CHRIS McKEEN/Stuff.co.nz Inside the cordon: Cousins rescue Edgecumbe residents by jetboat (This video was first published in 2017).

Systems most at risk were those with combined wastewater and stormwater pipes because sudden deluges could cause overflow, putting public health at risk.

Auckland has about 200km of such piping. The super-city was hit by several storms this winter that flooded hundreds of properties and caused hundreds of thousands of dollars in damage.

Pipes in some areas, such as parts of south Dunedin, were only designed for a one-in-two-year flood, leaving it highly vulnerable it to flooding as the runoff from the surrounding hills funnels in.

LAWRENCE SMITH/STUFF Major flooding in the Auckland suburb of New Lynn after a huge downpour in June.

In July, heavy rain infiltrated the southern city's wastewater system and contaminated water overflowed onto urban roads.

Local Government New Zealand (LGNZ) chief executive Malcolm Alexander said replacing aging 1960s piping and sewers alone could cost up to $100 billion.

Coastal areas were expected to bear the brunt of that because their pipes fed directly into the sea, leaving them vulnerable to corrosion from salt water and the possibility of back-flows as sea levels rose.

Stuff.co.nz Large parts of Dunedin are underwater as heavy rainfall cause chaos in the city.

In some locations this was already happening, spilling sewage and wastewater onto parks and roads during king tides, according to the report.

LGNZ president Dave Cull said improving waste and stormwater systems was one of the greatest challenges facing local councils nationwide.

Doing so would require more support from central government, given it will come at a significant cost, he said.

JOHN COSGROVE/SUPPLIED Flooding hits south Dunedin in 2016.

This year is already the most expensive year on record for insurance claims related to significant weather events, which have already hit $230m, according to the Insurance Council.

THE PROBLEM WITH OUR PIPES

Drawing on contributions from academics, scientists, government policy analysts and service providers, the report identified many areas where research was lacking.

These included some councils not knowing how old their piping was, or the state it was in.

The scale of the problem was put in perspective by figures that put the value of stormwater and wastewater assets at over $20 billion.

That included 24,000 kilometres of public wastewater networks, more than 3000 treatment plants, and over 17,000km of stormwater networks.

Many water and sewage treatment plants are located on floodplains or the coast due to the need to discharge to nearby sources of water.

But this means they are also exposed to sea level rises. If sea water gets in it can corrode machinery and disrupt some of the biological processes used at the plants, White said.

Sea levels have risen about 22cm over the past century, and those rises are expected to continue at a faster rateaccording to the Ministry for the Environment.

"Drought brings its own problems, disrupting gravity systems by slowing flow and leading to blocked pipes. Particularly lengthy droughts can also affect wastewater treatment processes, creating functional and safety concerns," White said.

Areas where drainage systems could not rely on gravity faced their own issues.

Increased extreme rainfall events were expected to overwhelm networks, restrict opportunities for maintenance, and increasing spill-over from sewage to stormwater systems.

Pipes feeding directly into the sea are also more vulnerable to liquefaction.