The Green Party says National is overseeing massive dairy farm conversions which could undermine the millions of dollars being spent on cleaning up the Waikato River.
The party says $210 million of taxpayer money is being spent on the clean-up while land in the Upper Waikato is being turned into a home for 30,000 more cows.
It says state-owned farmer Landcorp, is converting more than 25,000 hectares of pine forest to dairy.
Green Party co-leader Russel Norman said says figures from the Parliamentary Commissioner for the Environment showws the conversions are estimated to pollute the Waikato River with 700 tonnes more nitrogen per year.
He said it was ironic that National was proposing this on the very stretch of water it shot its rowing-themed election advertisement.