National leader Simon Bridges. Photo: NZ Herald

Plans to limit the amount of pollution farms can put into waterways are another "assault on the regions", leader of the opposition Simon Bridges says.

Bridges said that, like a ban on new oil and gas exploration, the Government's proposed limits on nutrient levels and farming intensification appear to have been promised without analysis about their potential impact.

"There seems to be a lack of any kind of comprehension that when farmers sneeze, we all catch a cold," Bridges told Radio New Zealand this morning.

"Whether in urban or rural areas, we are in this together. [Farmers] have been making a lot of environmental gains, but just culling the cows, when you haven't got a plan around that, and you don't have other viable industries coming up to take the slack is a very bad idea indeed."

Asked whether he opposed limits on how much polluted run-off could go into rivers, streams and lakes, Bridges said "not necessarily".

But he said there had been no economic analysis on the impact of the Government's policy and no detailed proposals for how industries could adjust to the new limits.

The policy was driven "by values, by vibe" and "not by hard evidence", he said.

Intensive farming, which grew significantly under the previous National–led government, has been blamed for the degradation of rural waterways.

Asked what National's plan for reversing pollution was, Bridges pointed to a multimillion-dollar fund for regional restoration projects.

Environment Minister David Parker said yesterday a new national plan would set a maximum limit on the amount of nutrients that could be lost from a farm into a waterway.

Farming intensity would also no longer be a "permitted activity". Both of the changes would be included in a revised National Policy Statement on Freshwater Management – a set of bottom lines which regional councils must use when setting their own policies.

Parker said while cow numbers had "already peaked" and were going down, in some areas, the number of cows per hectare was higher than the environment can sustain.

"That won't be done through a raw cap on cow numbers; it will be done on nutrient limits, the amount of nutrient that can be lost from a farm to a waterway, because it's not just a dairy cow issue."

The policy of setting nutrient caps was devised by the Land and Water Forum, a consensus-building group which includes environmental NGOs, scientistis, iwi and farmers.

Federated Farmers, which is part of the forum, called for a more "nuanced" approach than what Parker had suggested.

Water spokesman Chris Allen said limiting cows might help in some catchments, but each catchment is different in terms of soils, climate and rainfall.

Allen said there was also a need for more science - and for urban areas to address their own water pollution woes.