The Manawatu River is one of the most polluted in the Western world, according to new research.

The Manawatu tops a new pollution measurement of 300 rivers and streams across North America, Europe, Australia and New Zealand, research by the Cawthron Institute has found.

The waterway is fouled with treated sewage, industrial waste and farm runoff.

Under a system measuring oxygen changes in water, the Manawatu has by far the highest reading, almost twice as much as the next worst. The Manawatu measured 107. Anything over eight is considered indicative of an unhealthy river ecosystem. A measurement of 0–4 is considered healthy.

Roger Young, a freshwater ecologist with the Nelson-based independent scientific institute, was amazed at the readings from water samples taken at Hopelands, north of Woodville.

"To see a count of 100 was just ridiculous," he said.

Tests further south at Palmerston North were not as high but still indicated an unhealthy ecosystem.

Checks internationally found the closest pollution reading to be 59, for a site on a river near Berlin, downstream from a sewage outfall.

The Manawatu was affected by leaching farm nutrient and treated town sewage, Dr Young said.

"Agricultural use is most of it; nitrogen runoff, mainly."

Other factors were the shallowness and width of the slow-moving river, which exposed it to sunlight that encouraged algae growth.

Massey University ecologist Mike Joy said the research showed the river was "a basket case".

"I've been aware for some time that the river was a pretty sad case but even I was shocked at this research. It's not just farm nutrients; it's sewage, sediment and river modification issues with stopbanking."

Environment Minister Nick Smith said it was well known the Manawatu River was in poor shape.

In an Environment Ministry initiative this year on water quality for recreational use, it ranked 72nd out of 76 sites.

"No matter what way you look at it, the Manawatu River needs cleaning up."

Palmerston North Mayor Jono Naylor was concerned with the findings.

"No one likes the idea of having a polluted river running through their city."

The council had invested $15 million upgrading the wastewater system, so the quality of sewage discharged had improved.

The health of the river could affect swimmers' health, and warning signs were erected at some spots.

Horowhenua District Council admitted in September pumping 5.1m litres of partially treated sewage – including tampons, condoms and toilet paper – into the river over 48 hours in October 2007.

Dr Joy said: "At the high levels, you virtually have to get some in your mouth and you'll get sick."

Dr Young's research is to be presented to a panel which next week begins hearing submissions on Horizons Regional Council's One Plan.

The plan proposes tight rules to govern nitrogen leaching from farms. Farmers will get limits on the amount of nitrogen allowed in their soils, depending on their soil type.

Many farmers fear they will be forced to reduce cow numbers and take a substantial cut in earnings.

Federated Farmers regional president Gordon McKellar said the council was unnecessarily taking a big stick to farmers.

"Farmers are prepared to do a reasonable amount to manage their nutrients – after all, they've paid a lot of money for them and don't want to lose them – but regulation is not the way to get their support."

Horizons planning and regulatory manager Greg Carlyon said the nitrogen caps would get the river only halfway to swimming quality over 20 years.

WHAT IS POLLUTING THE RIVER?

* Farm runoff from fertilisers, and animal waste such as cow dung and urine, leach into the river.

* Treated sewage discharged by councils

* Treated industrial effluent including wastewater from Fonterra, New Zealand Pharmaceuticals, Tui Brewery

* Sediment washing into the river from overgrazed farms or eroding countryside alters the natural habitat for native bugs or fish

BY THE NUMBERS

* 25 resource consents to discharge into the Manawatu River have been granted.

* 75,600 cubic metres can be discharged daily.

* Palmerston North, Manawatu, Horowhenua and Tararua councils have consents to discharge treated sewage and wastewater.

* Horizons Regional Council has taken four prosecutions and 36 fine actions over illegal discharges this year.

* The top five resource consents for daily discharge:

46,600 cubic metres, Palmerston North City Council

24,000cum, Feilding sewerage plant

6370cum, Dannevirke

6000cum, Fonterra at Longburn

4600cum, Ashhurst