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According to the council's beach warning system, called Safeswim, Piha north and Bethells on Auckland's rugged West Coast have the best water quality of the city's beaches.
The beaches were rated the top two in the Auckland region with almost 100 per cent clean water last summer, with Cheltenham on the North Shore not far behind.
If we get a nice dry summer there will be plenty of swimming days.
Anecdotally, the most popular beaches in the city are Takapuna, Mission Bay, St Heliers and Cheltenham and regionally Orewa, Long Bay, Tāwharanui and Maraetai.
But popularity does not always translate to water quality. For example, Maraetai was predicted to have high water quality 95 per cent of the time last summer, whereas Takapuna came in at a relatively poor 70 per cent.
There are another seven beaches with historically high water quality, but because they were dropped from regular testing years ago and are only just being brought into the Safeswim testing programme, the council cannot say for sure they will match Piha north and Bethells.
The seven are Anchor Bay, Cheltenham, Devonport, Goat Island, Kendall Bay, Omaha and Tāwharanui.
Nick Vigar, the Safeswim project manager, said the West Auckland beaches of Piha north and south, Bethells and Karekare have excellent water quality and are swimmable nearly all of the time due to flow and tidal movement.
As you get closer to the city, water quality at beaches varied, Vigar said.
Of all the beaches on the North Shore, Cheltenham was the pick of the bunch with monitoring showing consistently high water quality from good water flow and a small network draining to it. Poor quality water only occurs there after comparatively heavy rain once or twice a summer, Vigar said.
But a few kilometres north at Takapuna, there were issues including illegal connections with the ageing stormwater and wastewater networks that need fixing, and the water quality there was poor, he said.
"We know that Takapuna beach has a particular circulation that tends to hold contaminants in there, whereas some other beaches may flush a little more easily," Vigar said.
Generally speaking, if there is no rain for several days most city beaches are fine for swimming. But heavy rain is a curse for swimming, causing stormwater and wastewater pipes to reach capacity and overflow.
"If we get a nice dry summer there will be plenty of swimming days. If we get a lot of rainfall, not so much," said Vigar.
Other city beaches with consistently high water quality are Beach Haven, Māngere Bridge, Castor Bay, Pt England and Pt Chevalier. Further afield, Wenderholm, and Onetangi and Palm Beach on Waiheke Island, have a good record.
When it comes to Auckland's worst beaches for water quality, there was good news in October when the council lifted long-term public health warnings at four beaches in the Manukau Harbour.
The four beaches - Armour Bay, Taumanu East, Clarks Beach and Weymouth - are now considered low risk for most of the time, with high-risk periods brought about by rain or storms, or damage to infrastructure.
Vigar said a lot of work had gone into identifying and fixing wastewater contamination of stormwater discharges at Weymouth beach and work at Clarks Beach by community volunteers collecting water quality samples quickly helped council to develop an accurate water quality model.
Long-term warnings not to swim remain in place at Cox's Bay, Fosters Bay, Green Bay, Laingholm, Meola Reef, Titirangi Beach, Wairau outlet and Wood Bay.
Council has also abandoned orange lights as part of its warning system, saying beaches are either swimmable or not, with no room for a "swim with caution".
Safeswim, run by council and Auckland Regional Public Health and Surf Life Saving's northern region, said people did not know what the orange light meant and Vigar said many people indicated last summer that they regarded the amber light as a public health warning, a high risk.
There was no different health advisories for green and amber and no different advice from a public health perspective, he said.
For the latest information about water quality at Auckland beaches go to: safeswim.org.nz