New Zealand's clean and green reputation is under threat from rising water pollution levels at a time when water pollution in other developed nations is falling.

A report from the Organization for Economic Co-operation and Development (OECD) described New Zealand's water quality as "good" compared with most OECD countries, but said it was deteriorating, according to an OECD official.

The deterioration was largely due to pollution from agriculture, Dr Kevin Parris, of the OECD Trade and Agriculture Directorate in Paris, told the 15th International Conference of the International Water Association Diffuse Pollution Specialist Group (DIPCON) conference, in Rotorua Friday.

New Zealand's agriculture, a vital part of the country's economy, was exerting more pressure on its environmental systems as it expanded, said Harris.

Agriculture contributed to water pollution through excess nutrients, pesticides and other pollutants.

Agricultural nutrients were a major pressure on river, lake and marine water quality, and the OECD assessed the pressure through nutrient balances, said Parris, the plenary speaker at the conference.

"We look at all the nitrogen and phosphorus going into the system, mainly from livestock manure and fertilizers and calculate how much nutrients are used to grow crops and pasture," said Parris.

"In most situations there is a surplus of nutrients to crop and pasture requirements, which places stress on the environment ( soils, water and the air). In 2000, the average for New Zealand was around 35 kg of nitrogen per hectare. By 2008 it was about 45 kg per hectare. In 10 years it has gone up 10 kg."

In 2000, the average for the 34 OECD countries was 80 kg per hectare, but in 2008 it had dropped to 65 kg per hectare.

Among the worst ranked was the Netherlands with a surplus of over 200 kg per hectare.

"They have a lot of cows and not much land," said Parris.

With the world needing to produce 50 percent more food by 2030 and needing to double production by 2050, the expansion of agriculture would further stress water quality in New Zealand and worldwide.