Rivers in Northland and Auckland are the dirtiest in the country - with fewer than one in four suitable for swimming, according to a new report - the first ever nationwide snapshot of the state of the country's waterways.
The joint report by regional councils, the Ministry for the Environment and the Ministry for Primary Industries, detailed work underway to meet the previous government's freshwater target, which was to make 90 percent of the country's larger rivers and lakes swimmable by 2040.
In Northland, just 24 percent of rivers and 67 percent of lakes were clean enough to swim in, compared with the national average of about 70 percent for both.
In Auckland, 23 percent of rivers (by length) were swimmable and 97 percent of lakes.
Economic analysis found the work councils were already doing (including fencing rivers and planting) would raise swimmability nationwide to 76 percent, which was still well short of the target.
The projected cost was than $217.3 million a year - of which $135m would be on the rural sector and $82m on urban ratepayers.
Councils have until the end of this year to discuss the draft regional targets with their communities and finalise them.
Local Government New Zealand President Dave Cull said all councils were committed to working with central government and communities to meet water quality expectations but for some it was going to be "a challenge".
"There will be costs associated with achieving improving freshwater quality, and we look forward to continuing to work constructively with the government to make sure councils have the resources to do this."