Pilot Bay has emerged as the surprise result in this year's National Water Quality trends report, with swimmers told to show caution in the family-friendly Mount Maunganui swimming spot.

The analysis led by the Cawthron Institute Freshwater Group graded the overall bacterial risk as medium at Pilot Bay. Frequent tests since October 2017 showed enterococci levels per 100ml of water usually below 10, but with six spikes of 25 to 125 enterococci per 100m.

The caution advisory meant it was ''usually suitable for swimming but younger children and older people may be at increased risk at times''.

Mount Main Beach was rated as being suitable for swimming, along with the main beach at Fergusson Park in Matua and Rangataua Bay in front of the marae.

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

Water quality at swimming spots around the Western Bay of Plenty. Image/LAWA
Water quality at swimming spots around the Western Bay of Plenty. Image/LAWA

Caution was advised for the Waimapu Estuary where the testing point was in front of the Silver Birch motor camp.

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 continue to steer clear of the other regular black spot, Welcome Bay's Kaiate Stream at Kaiate Falls.

Kauri Point's small beach and the Henry Rd ford on the Uretara River in Katikati were rated as unsuitable for swimming.

Unsuitable for swimming were sites where there may be a health risk and were not considered suitable for swimming most of the time.

Waihi Beach was judged suitable for swimming but cautions were advised for the Three Mile Creek section of Waihi Beach, Tanners Point and the Tuapiro stream swimming hole at McMillan Rd.

Omokoroa's beach was judged low risk and suitable for swimming, together with Maketu Beach and Pukehina Beach at the estuary boat ramp.

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.