Green Party water spokesperson Catherine Delahunty says new reports from the Parliamentary Commission for the Environment indicate dairy intensification is causing increasing damage to New Zealand's waterways.
The discharge of pollutants from dairy operations means the water quality of many rivers and streams is continuing to decline.
"While we welcome the good work the Parliamentary Commissioner Dr Jan Wright has done on this report, the findings are grim reading," says Ms Delahunty.
"We would like to see the Commissioner call for a moratorium on further dairy conversions in light of the findings that dairy is causing further deterioration to waterways."
Water quality in New Zealand: Land use and nutrient pollution update report shows that between 2008 and 2012, the Waikato lost 18,700 hectares of plantation forests. Another 28,400 hectares of land became dairy farms. Delahunty says these conversions are illogical, because forests prevent some unwanted substances from leeching off into streams.
"It makes no sense to be converting forested land in the upper Waikato to intensive dairy farms such as state-owned enterprise Landcorp are doing. By putting another 29,500 cows on the land, Landcorp are not only harming the river, they are undoing the good work farmers downstream have done mitigating pollution."
The report found that in total, New Zealand lost 9,600 hectares of plantation forests, but increased dairy by 157,900 hectares. It also noted that planting will not solve the fundamental problem of a large number of cows, because dairy generates so much waste.
"It's another disaster for the environment, and we're calling for a moratorium on these conversions to stop further pollution," Ms Delahunty said.