The Government's planning the biggest water quality reforms in almost 30 years.

Agriculture Minister Damien O'Connor and Environment Minister David Parker on Thursday unveiled new proposals for a swathe of major changes intended to clean up New Zealand's rivers, streams and lakes.

The shake-up is the biggest to the standards since the introduction of the Resource Management Act more than two decades ago and aims at quickly halting the degradation of freshwater, improving conditions within five years and fixing them in a "generation".

"If we don't fix things now they only get worse and will be more expensive to fix," Parker said.

Much of the debate about river quality has centred on the effects of nitrates coming from growing and intensifying dairy farming in recent decades - and particular livestock waste and fertiliser putting nitrate into waterways, along with pollution from sewage and urban growth.

In one of the most significant changes, the Government has proposed from June next year restricting further intensification of rural land, putting the brakes on some development.

It will mean dairy conversions over 210 hectares or irrigation schemes would only go ahead if they could prove they wouldn't increase pollution.

The restrictions would stay in place until at least 2025, by when regional councils would have to have new rules in place based on a new "National Policy Statement" on freshwater also announced on Thursday.

Other major changes farmers may face include:

• the introduction of new nitrogen caps for waterways in some regions
• new farm planning requirements that consider water quality
• stricter rules to keep stock away from waterways
• new restrictions and rules for winter grazing, feedlots and stock holding areas
• protections designed to stop development on wetlands

The recommendations would also require councils to set significantly tougher limits for nutrients in waterways around the country.

The technical group advising the Government said fewer than half of the country's 16 regional councils had set nutrient limits in some catchments using current rules, and that the existing "bottom lines" weren't good enough.