The Government has set a target of making 90 per cent of New Zealand's rivers and streams safe for swimming by 2040.
That means all waterways will have to meet the safe standard for E.coli contamination 80 per cent of the time.
Announcing the target in Auckland today, Environment Minister Nick Smith said 72 per cent of New Zealand's were currently "swimmable", according to a standard used in Europe and the US.
Raising that level to 90 per cent would require cleaning up 10,000km of waterways, and would cost Government, farmers and councils an estimated $2 billion. Farmers would have to build 56,000km of fencing.
Responding to the announcement today, the Green Party said the Government had "shifted the goalposts" for determining whether a river was swimmable.