Concerns for water quality around New Zealand ahead of summer swim season

As the warmer summer weather returns, so does the desire to swim.

But a recent sewage leak and multiple heavy rainfall events mean your favourite swimming spot may harbour hidden secrets.

Corsair Bay is a favourite swimming spot for those living in Canterbury, but right now it's too unsafe.

"There is faeces in the water that has come from mammals," Environment Canterbury science director Tim Davie said.

"The one that we're most concerned about is humans and that's what we've found in Corsair Bay, human waste."

Contaminated water can make you seriously ill. Corsair Bay isn't the only unsuitable recreational swimming spot in Canterbury.

Several sites within Lyttelton and Akaroa Harbours, including Akaroa Main Beach, Rāpaki Bay, Sandy Bay, Diamond Harbour Beach and Purau Beach, are also unsafe.

"In the case of Corsair Bay it's people's housing that we are particularly worried about and human waste coming through the sewer system," Davie said.

Canterbury is just a snapshot of the state of our waterways all over the country.

Most of Auckland's beaches were out of action for weeks after a burst sewer and a 13-metre-deep sinkhole in Parnell allowed millions of litres of wastewater to flow into the harbour.

Although several beaches have since been cleared it's a stark reminder to check before you swim.

Long-term water grades have just been released ahead of summer.

"It shows that there's certainly concerns at some of our swimming spots around the country and our rivers are probably worse than our lakes and coastal areas," Freshwater Ecosystems manager and scientist with the LAWA project, Dr Roger Young said.

Sixteen percent of our rivers are unsuitable for swimming and caution is advised for 14 percent. 

Meanwhile 10 percent of coastal areas are either unsuitable for swimming or caution is advised and lakes are deemed the safest, with just 8 percent either unsuitable or caution is advised.

"Lowland waterways and coastal areas near urban development are the areas where you need to be a bit more cautious," Dr Young said. Especially after rain. 

Testing will continue throughout summer but overall improvement in water quality is likely to take much longer.