Chamberlains Ford, on the Selwyn River, is the only monitored swimming spot between Ashburton and Kaikoura with "good" water quality.

More Canterbury rivers are becoming unsafe to swim in each year, despite a major target to make rivers more swimmable.

This summer, 64 per cent of river sites monitored by Environment Canterbury (ECan) have been deemed safe for swimming, down from 74 per cent five years ago.

When the Government replaced the elected council with commissioners in 2010 – supposedly because it was failing to meet water targets – the commissioners aimed to have 80 per cent of rivers swimmable by 2015.

NIWA River flow data from NIWA - July on the right and September on the left - shows some Canterbury rivers had flows "far below" normal levels, even in the middle of winter.

It revealed in August it had missed its deadline, and the number of swimmable rivers had dropped to 67 per cent. It now says that figure has fallen again, to 64 per cent.

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"There has been some recent decline in the monitored sites," an ECan spokesman said.

He said despite the drop, it was higher than in 2004, when only 53 per cent of sites were recommended for swimming.

Current data showed sites with "poor" or "very poor" ratings – which are not deemed safe for swimming – included every monitored site on the Waimakariri and Hurunui rivers, as well as most sites along the Selwyn and Ashburton rivers.

The only monitored site between Ashburton and Kaikoura with "good" water quality for swimming was Chamberlains Ford, on the Selwyn River.

Green MP and former ECan councillor Eugenie Sage said the dwindling number of swimmable rivers was not good enough.

"Previous generation Cantabrians have been able to fish and swim in rivers on their doorstep, and now people are denied that opportunity. It's a result to be ashamed of."

She said with hot temperatures and low river flows due to El Nino, it was likely algae blooms would make more sites unswimmable.

"When there is a health risk for people walking dogs or people going swimming, it shows how we desperately need a major change in water quality management - strong, national bottom lines for rivers and lakes that are swimmable, not wadeable."

With another hot, dry summer ahead, toxic algae blooms caused by low water flows were adding further concerns around swimming.

At the close of last summer, 11 sites in Canterbury had warnings for cyanobacteria, which was a hazard to both people and animals.

Already this summer 15 sites had such warnings, including popular camping and swimming spots Coes Ford and Evans Crossing, where toxic algae had been detected in "significant quantities".

Canterbury medical officer of health Dr Alistair Humphrey said exposure to cyanobacteria could cause "skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips".

It could be fatal to stock, horses and dogs. Low river levels were a major contributor to algae blooms.

Data from NIWA shows some Canterbury rivers - particularly the Selwyn river - have had flows "far below" normal throughout the year.

ECan commissioner David Caygill said rules around taking water were being treated seriously, as the region prepared for ongoing drought-like conditions.

"Environment Canterbury takes the efficient use of water seriously, particularly in these times of extremely dry weather.

"Drought conditions are expected to continue and users will keep facing restrictions. People can't use what isn't available."

Irrigation restrictions were in place in some areas and would be monitored to ensure compliance, he said.

An ECan spokesman said people should not be wary about swimming in rivers, as water quality in Canterbury was generally good.

They should look at the council's website to check water quality at each site.

Beaches, on the other hand, were almost unanimously safe for swimming, and had marked some of the best water quality seen in several years.

All monitored coastal swimming sites except one – the Avon Heathcote Estuary at Humphreys Drive – were safe for swimming.

Popular swimming sites such as Corsair and Cass Bays and New Brighton and Sumner beaches had "good" water quality.

Sites with "very good" water quality included Taylors Mistake, Spencerville Beach and Woodend Beach.

EL NINO

A cruel twist to the searing El Nino summer may shock those dipping their toes into the sea for the first time.

A side-effect of El Nino conditions in New Zealand were cooler ocean temperatures, which were already happening in Canterbury.

"The sea is running colder than it should be, and it looks like it's around a degree cooler than usual for the time of the year," MetService meteorologist Georgina Griffiths said.

"It doesn't sound like much, but it is noticeable."

The hotter the air got, the colder the sea would feel. During Monday's record-breaking 36°C day, the ocean temperature was just 13.5°C.

Canterbury's seas would likely remain cooler than usual for the rest of the summer, Griffiths said.

"It's a real El Nino thing, the cool seas around New Zealand in the run-up to Christmas ... off the Canterbury coast, seas will end up half a degree cooler than where they would normally be by the end of summer."