Owhiro Bay's water quality has been graded the worst of Wellington city's beaches, with five others in the region also classed as unsuitable for swimming.
During the summer months the Greater Wellington regional council and the relevant local councils jointly monitor water quality at 61 recreational beach spots weekly.
The latest annual report, published in December, rates the sites in three categories - ongoing concern, sometimes contaminated after rainfall, and normally safe and clean.
The worst sites are South Beach at Plimmerton; Porirua Harbour at the rowing club; Titahi Bay at South Beach Access Rd; Lowry Bay at Cheviot Rd; Rona Bay at Cliff Bishop Park; and Owhiro Bay.
"These sites are of concern to us as it means they are not suitable for swimming," said Summer Greenfield, an environmental scientist at the regional council. "Over the course of this summer we are planning on investigating the source of contamination."
It was important to find out if the cause was of human origin, such as a failing in the stormwater or sewerage network.
The readings monitor specifically for the faecal indicator bacteria enterococci, which is normally found in the faeces of people and animals, and has been known to cause human disease or infection.
Brooklyn father David Lloyd was out paddling at Owhiro Bay yesterday with his son Sam Pope, 1, and said the beach's grading was not of concern. "[The warning sign] has been there for a couple of years."
He said his family chose Owhiro Bay after finding nearby beaches covered by fog. They stuck to the shallows and would never take seafood from a place with such a rating.
Wellington's water quality is not all bad news, however. At the end of the 2011-12 bathing season, 64 per cent of coastal monitoring sites in the region were rated good or better.
Mrs Greenfield said there were a few coastal sites in the Wellington region which were graded as very good. The best spots for swimming in terms of water quality were Princess Bay, Breaker Bay and the beach at Castlepoint.
The council advised caution at spots rated fair - particularly up to two days after significant rainfall - and that people check its website for updated readings.
For updates throughout the summer, visit gw.govt.nz/recreational-water-quality.
- The Dominion Post
Is this the best summer ever?Related story: Wellington sizzles in January heat