Most New Zealand rivers were unsafe to swim in last summer.
Figures from local authorities obtained by the Green Party show two thirds of the waterways monitored were ranked too dirty to dive into during last year's holiday season.
Of the sites tested, 63 of the 166 were ranked as poor and 46 as very poor.
Those ratings mean swimmers are at risk of getting gastro-intestinal illnesses and infections.
The data excludes rivers in Auckland, Waikato, Northland and the West Coast, which use a different monitoring standard.
But figures from the Land, Air, Water Aotearoa website show e coli levels in Auckland rivers are in the worst 25 percent of the country, and Waikato rivers fell into the worst half.
Green Party water spokesperson Catherine Delahunty said the number of rivers failing to reach a safe standard has jumped from just over half, to two thirds, in three years.
She said the standards introduced last year are too low and have not stopped rivers deteriorating.
"Although we don't have the figures for this summer, last summer was worse than the summer before. With more than 6 million dairy cows on the land, and with a lot of waterways and rivers being polluted from sewage - we're really not improving, and there's a lot of work to be done."
It should be mandatory, she said, to fence off livestock from rivers, and small towns should be given more help to keep their rivers clean.