It's inevitable the Waikato River will be harmed if the state farmer Landcorp goes ahead with its plans to convert massive tracts of forestry in the region to dairy farms, MPs have been told.

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They have been presented with a Green Party petition signed by more than 8000 people calling on Landcorp not to proceed with its conversions.

Green MP Catherine Delahunty said introducing tens of thousands more cows into the Waikato River catchment would come with a big environmental cost, and the petition showed many people were opposed.

"It (the petition) calls for Landcorp to stop converting to dairy at a time when our rivers are extremely stressed, so 8200 New Zealander said let's have a moratorium," she said.

Massey University freshwater ecologist Russell Death told MPs many Waikato dairy farmers were concerned about Landcorp's intentions.

"In the Waikato it seems really ironic that downstream, a lot of the farmers are worried about the river, that they're putting in nutrient management plans to improve things," he said.

"But now they're seeing upstream that Landcorp's doing the opposite, and they're thinking 'well what's the story here'."

Professor Death said it was inevitable the Waikato River would suffer from the influx of additional cows, which he said would increase nitrogen leeching into the river.

He said while he had previously worked closely with the Ministry for the Environment, that had ceased under the current Government.

Professor Death said the Ministry no longer wanted to hear what he had to say.