Much of New Zealand's more than $20 billion of wastewater and stormwater systems aren't designed to withstand the potentially extreme impacts of climate change, researchers say.
The latest study out of the Deep South National Science Challenge - a research collaboration with a strong focus on climate change - has looked at the threat that extreme weather events and rising seas could have on our sewer networks.
The recent Edgecumbe floods, for example, saw raw sewage floating through the streets, making the clean-up extremely challenging.
Six months later, 500 houses are still uninhabitable and flood-proofing the town remained a distant goal.
"The way climate change is predicted to affect our stormwater and wastewater will have a considerable impact on many aspects of New Zealand life, including health, disaster resilience, drinking water, ecology, and transport, not to mention how flooding or infrastructure failure will impact on communities," said the report's co-author, Professor Iain White of Waikato University.
New Zealand's infrastructure included 24,000km of public wastewater networks with more than 3000 pump stations, and more 17,000km of stormwater networks.
Much of it, however, was not designed for the challenges climate change would bring, from sea level rise to the predicted changes in precipitation frequency and intensity.
"For example, in many local water systems, roads are designed to be used as a secondary stormwater routes in extreme flooding.
"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."
The increase in extreme rainfall events would also add stress to the system by overwhelming the networks, restricting opportunities for maintenance, and increasing the occurrence of infiltration of wastewater into stormwater.
"We already know that sea level rise will affect all coastal infrastructure, and as many of our water networks use gravity to discharge to water bodies, the most costly areas of the network are often located in low-lying areas or on the coast," White said.
"From this, increasing sewage overflows, pipes corroded by salt water, and exposure to liquefaction are all more likely."
An increase in the number and frequency of coastal storms would hit coastal infrastructure in particular, causing increasing inundation, physical damage, and electrical failure at treatment plants.
"It's not just too much water, though," White said.
"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."
The new report suggested areas for further research - with the first priority being to quantify the precise risks to these systems.
"Once we have this knowledge we need to consider the most appropriate adaptation response and which practical solutions may help reduce these impacts.
New Zealand and climate change
• Under present projections, the sea level around New Zealand is expected to rise between 30cm and 100cm this century. Temperatures could also increase by several degrees by 2100.
• Climate change would bring more floods; worsen freshwater problems and put more pressure on rivers and lakes; acidify our oceans; put even more species at risk and bring problems from the rest of the world.
• Climate change is also expected to result in more large storms compounding the effects of sea-level rise.
• New Zealand, which reported a 23 per cent increase in greenhouse gas emissions between 1990 and 2014, has pledged to slash its greenhouse gas emissions by 30 per cent from 2005 levels and 11 per cent from 1990 levels by 2030.
• The new coalition Government has promised greater action, with a proposed new independent Climate Commission and Zero Carbon Act and goals for a carbon-neutral economy by 2050 and 100 per cent renewable energy by 2035.