Sewage overflows have been blamed for Tauranga's family-friendly swimming spot of Pilot Bay getting a cautionary ranking in New Zealand's annual water quality report.

The analysis led by the Cawthron Institute Freshwater Group grades overall bacterial risk at swimming spots and has identified Pilot Bay as having medium risk.

It was based on three years of water testing, with the highest level of enterococci at Pilot Bay reaching 660 per 100ml over water. Enterococci indicated the presence of potential disease-causing faecal material in water.

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The report showed how there had been six occasions since October 2017 when enterococci spiked at up to 125, amid an otherwise flat result of no more than 10 enterococci per 100m of water.

The caution advisory meant it was usually suitable for swimming but there could be an increased risk at times for the very young, very old or people with compromised health.

Tauranga City councillor Steve Morris said he could not recall Pilot Bay being anything other than a suitable place for swimming, with the exception of the times when there were wastewater overflows from blockages in the sewerage system.

Councillor Leanne Brown said wastewater overflows might have tipped the scales but otherwise she thought Pilot Bay was a very safe swimming spot for the majority of the time. Tidal flows constantly changed the water.

''This has not changed my level of confidence to swim there. I would definitely not view Pilot Bay as an unsafe place to swim.''

Mount Mainstreet manager Ingrid Fleming said she was amazed at the cleanliness of Pilot Bay, considering how much shipping went past every day.

''When you look at the water and the clarity and how clean it is, we are probably right up there in the world for keeping our water clean regarding a port,'' she said

''We would all rather have a top score, but we should recognise how lucky we are with such a busy port.''

Mount Main Beach received the top rating of being suitable for swimming, together with the beach at Matua's Fergusson Park and Rangataua Bay in front of the marae next to Maungatapu Bridge.

Kopurererua Stream at McCord Ave in the Judea industrial area, a popular place for kayak slalom training, was graded as suitable for swimming.

Cautions were advised for the Waimapu Estuary where the testing point was in front of the Silver Birch Holiday Park.

There was no change in the usual ''unsuitable for swimming'' status of the Wairoa River below McLaren Falls and at the SH2 bridge.

Swimmers were also urged to steer clear of the Bay's other pollution hot spot, Welcome Bay's Kaiate Stream at Kaiate Falls.

Unsuitable for swimming was described as being sites were there could be a health risk and were not considered suitable for swimming most of the time.

The small beach at Kauri Point and the Henry Rd ford on the Uretara River in Katikati were also graded as being not suitable for swimming. Kauri Point was getting a sewage scheme to replace septic tanks.

Waihi Beach's main flagged beach was judged as being suitable for swimming. Cautions were advised for the Three Mile Creek section of Waihi Beach, Tanners Point and the Tuapiro Stream swimming hole at McMillan Rd.

Omokoroa Beach was ranked as suitable for swimming, with Maketu Beach and Pukehina Beach at the estuary boat ramp also rated as suitable for swimming.

Meanwhile, New Zealand's river water quality trends showed cause for optimism. All river water quality parameters monitored over 10 years showed more sites were improving than deteriorating.

This national picture was welcomed by scientists and local government who pointed to freshwater ecosystem management practices as likely contributing to the progress.

Cawthron Institute Freshwater Group Manager and Ecologist, Dr Roger Young said the overall picture was encouraging.


Coastal beaches swim grading:
Suitable for swimming: Faecal indicator bacteria less than 140 enterococci per 100m of water.
Caution advised: Faecal indicator bacteria more than 140 enterococci per 100m of water.
Not Suitable for swimming: More than 280 enterococci per 100ml of water.