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Water New Zealand say messages about preventing sewage blockages not getting through to Kiwis.

People using their sinks and toilets as "waste disposal units" have been responsible for many of the sewage overflows during the dry weather this summer.

So says Water New Zealand, which has just published its latest performance comparison report, the 2017/18 National Performance Review (NPR), which looks at the state of our drinking, storm and wastewater assets.

The majority of the overflows have been caused by blocked pipes, an all too common outcome of people treating their toilets and sinks as waste disposal units, the review says.

The NPR found that since 2015-16 the number of sewage overflows occurring during dry weather blockages has increased five-fold.

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"This clearly suggests that messages about binning wet wipes and other non-flushable items are not getting through to enough people, " says the report's author, Water New Zealand Principal Data Scientist, Lesley Smith.

"Wastewater operators commonly find not only wet wipes but also paper napkins, feminine hygiene products and kitty litter clogging up our networks.

"This is having a huge impact on the wastewater network, which has only been designed to accept our pee, poo and toilet paper. The outcome of this behaviour is that sewers block up, causing sewage to overflow into rivers and the ocean.

"Not only does this create and expensive clean up, but it damages our environment and public health can be put at risk as well."

She says that the number of overflows caused by pipe blockages increased from 0.4 per 1000 properties in 2015/16 to nearly two per 1000 households this past year.

"Last year there were 1,642 dry weather overflows caused by blockages compared with only 177 due to mechanical issues.

"Many people are simply not aware of the huge problems that putting wipes and other non-flushable products into sinks and toilets can cause our environment and sewers."

The report follows that of 2018, released in April last year, which reported a 379 per cent increase in the number of sewage overflows to the environment – but caused by wet weather and extreme weather events.

And unless big steps are taken to significantly upgrade storm and waste water infrastructure, sewage will continue to spill into our beaches and waterways, Water New Zealand says.

A report by the sector group that benchmarked council performance around drinking water, waste water and stormwater revealed there was a 379 per cent increase in the number of sewage overflows to the environment caused by wet weather.

Water New Zealand's group chief executive, John Pfahlert, said at the time the impact of climate change meant that without a concerted focus, more and frequent sewage overflows were likely to become the norm.

"Data in the [2018] NPR reveals that in some networks the volume of sewage in wet weather can be more than 10 times the volume as in dry weather," he said. "When it rains stormwater makes its way into the sewers in a variety of ways such as cracks in ageing pipes or gully traps being incorrectly hooked up into the wastewater system and so on.

Auckland Council was spending $1.2 billion on its new Central Interceptor to separate wastewater and stormwater flows. It was expected that will reduce the annual overflow volume to its harbours and waterways by 80 per cent.

"But it's far from just an Auckland problem. The NPR data revealed that 35 out of 41 authorities in the review report some level of sewage overflows caused by wet weather."