The invisible contamination at New Zealand's beaches

As the sun broke after the many stormy days the North Island saw this summer there is a natural desire to head to the beach.

But that is often not the best idea. Heavy rain in New Zealand's major cities causes stormwater to run off the roads and footpaths into the sea.

Those roads and footpaths contain many pollutants, including bird and animal faecal matter that flows with the rainwater into our seas.

On top of that, heavy rain can often overload our wastewater systems, meaning human faecal matter will be dumped into the sea.

Safeswim monitors the contamination levels of our beaches and over the summer has regularly posted warnings not to swim at some of Auckland's most popular beaches.

This summer Safeswim's map regularly had red or black flags indicating it was not safe to swim.

The black pins on the website mean the beach has been affected by an overflow of wastewater. It is the highest warning and people are warned to not swim in those particular areas.

"It is likely Aucklanders will see more black pins following wet weather this summer," Safeswim told Newshub in January.

The red pin represents a high likelihood of the water becoming contaminated and people becoming sick if they swim in the water.

The problem was not isolated to Auckland, Wellington also experienced contaminated water this summer.

Heavy rain in January forced 38,000 litres of waste to overflow into the ocean just south of Lyall Bay.

Climate scientist James Renwick told Newshub in January Aotearoa's infrastructure can't cope with deluges.

"Our water infrastructure in a lot of places is 100 years [old] and is built for the climate we used to have, not the climate we have now."

The only real way to check if the beach is safe to swim at is the Safeswim website, but many people are unaware of the issues and the water can be full of swimmers even with warnings in place.

The invisible contamination at New Zealand's beaches

Auckland Council Head of Planning for Healthy Waters Nick Vigar told AM you can't tell from looking at the water if it is contaminated.

AM host Ryan Bridge gave Vigar three samples taken from Auckland Beaches, including one from Cox's Bay, which has a permanent no-swim notice.

"If there is one thing I have learned you can't tell by looking at the water," Vigar said.

"Cox's Bay is part of the Western isthmus which is a part of Auckland with combined wastewater and stormwater networks. So a lot of spills.

"There are plans in place to fix the problem but right now don't swim at Cox's Bay."

Vigar said the main issue after heavy rain is faecal contamination from animals, birds and even humans.

"What we are talking about is faecal contamination, or poo, in the water.

"That can be from a variety of sources. It can be human but a lot of it is from animals, especially in more rural situations it will be from livestock."

He said the main cause of contamination is stormwater, where heavy rain washes the contents of the footpaths and roads into the sea.

"You see what's on the footpath, bird and dog poo. So when the rain washes that off it heads into the sea."

"We are never going to fully stop that. On top of that though in the cities, there are wastewater  networks that get overwhelmed."

Vigar told AM they routinely test for E.coli and enterococci, two bacteria that are indicators of faecal contamination, but that doesn't identify the source.