Popular swimming spots in lakes and rivers have high levels of bacteria that cause diarrhoea or infection, a new report shows.
Of 300 popular swimming spots tested regularly by councils over the past two summers, just 58 per cent were suitable for swimming almost all of the time.
The rest had too much faecal matter in the water at least 5 per cent of the time - and one spot never met the guidelines for safe swimming.
Environment Minister Nick Smith said the results were unacceptable and that farmers, councils and the Government needed to do more.
The report, which showed no significant change in national fresh-water swimming water quality over the past six years, found 8 per cent of swimming spots breached health guidelines at least a quarter of the time.
Northland region had the most spots - eight out of 20 - that were consistently too polluted to swim in.
Piha Lagoon - popular among parents who want to let their children cool down away from treacherous west coast waves - was Auckland's worst spot, meeting health guidelines less than 47 per cent of the time.
Elsewhere in Auckland, Bethells Lagoon met guidelines 89 per cent of the time, and Lake Pupuke and Lake Wainamu were safe for swimming every time they were tested.
The worst spot in the country was Waimoku Stream at Oakura Beach in Taranaki. It breached health standards at every test.
To meet health guidelines, water is tested weekly over summer to see if it is under the threshold of 550 E. coli per 100 millilitres of water.
The swimming water report was released alongside a second report showing water in dairy farming catchments had higher levels of faecal contamination than "average" lowland farming areas.
Forest and Bird Society spokesman Kevin Hackwell said the latest report showed criticisms of the voluntary Dairying and Clean Streams Accord aimed at cleaning up fresh water between Fonterra, councils and Government ministers were "spot on".
The society and the Fish and Game Council have argued the accord has done little to stop effluent ending up in streams and rivers, in part because it does not set water-quality targets.
Mr Hackwell said it was not surprising swimming spots in Northland were polluted because the holiday region had one of the highest rates of dairy farms that did not comply with effluent discharge rules.
Dr Smith said a "quiet revolution" in farming practices had cut down effluent pollution from cowsheds.
But he said more work was needed to stop cows walking through streams, increase stream-side planting and make better use of new fertilisers that could decrease nitrogen pollution.
All swimming spots tested in the Waikato met guidelines, including Lake Taupo and the Waikato River.
Waitakere City Council spokesman Glyn Walters said the council was working on cleaning up Piha Lagoon but no single source was to blame for its water-quality issues.
Runoff from roofs, driveways and roads, dead wildlife and rotting vegetation could push up levels of bacteria in the water.
* Swimmers beware
Percentage of water samples complying with health standards, summers of 2007-8 and 2008-9:
Waimoku Stream at Oakura Beach, Taranaki:0
Kaiapoi River at boat ramp, Canterbury:10
Lyell Creek lagoon, Canterbury:20
Otamure Bay stream, Northland:24.7
Langs Stream, middle beach, Northland:35.9
Waiharakeke River, Northland:37.5
Piha Lagoon, West Auckland:46.6
Whangarei Falls, Northland:53.2
Lake Taupo, Lake Rotorua, Lake Pupuke on Auckland's North Shore and most swimming spots on the Waikato River met health standards every time they were tested.