The Government has set a target of making 90% of New Zealand's rivers and streams safe for swimming by 2040.
However, it has changed the guidelines for what is considered suitable for swimming. Rivers will now be rated in "excellent" condition if they contain 540 parts of E.Coli per 100 millilitres of water - double the previous standard.
Announcing the target in Auckland today, Environment Minister Nick Smith said 72% of New Zealand's were currently "swimmable", according to the standard used in Europe and the US.
Raising that level to 90% would require cleaning up 10,000km of waterways, and would cost Government, farmers and councils an estimated $2 billion.
It would be backed by national rules which require stock to be kept out of waterways and will make regional councils tighten their rules on sewage discharges.
Dr Smith described the target as "ambitious" and said it recognised that New Zealanders expected to be able to "take a dip in their local river or lake without getting a nasty bug".
At present, the minimum standard for water quality in New Zealand is "wadeable".
The standard will now be changed to "swimmable" - despite previous statements by Smith that this was not practical.
"It will return our rivers and lakes to a standard not seen in 50 years while recognising that our frequent major rainfalls mean a 100% standard is not realistic," Dr Smith said.
The latest reports on water quality in New Zealand said around 60% of rivers, streams and lakes were not safe for swimming.
But Dr Smith said this was misleading because councils focused their monitoring on higher-risk sites.
The Government has used new data and monitoring to determine that 40% of waterways were not suitable for swimming.