New Zealand cannot afford to wait to address the problems with fresh water, Ministry for the Environment head Vicky Robertson says.
A landmark report from the ministry, released today, describes serious challenges facing the country's rivers and outlines how fresh water is under increasing pressure from agricultural and urban areas.
"Agriculture intensification does have an impact on water degradation ... what we can't say is which part of agriculture is having that impact" - Vicky Robertson
The report found freshwater biodiversity was declining and 72 percent of native fish were threatened or at risk of extinction - as were about a third of freshwater plants and invertebrates.
Nutrients from rural and urban waterways were a growing problem and the report showed nitrogen was getting worse in more than half the country's monitored rivers.
Ms Robertson said, while the data was not perfect, there was enough to show that the country needed to act to clean up waterways and to make different choices.
She said urban waterways were the most polluted, but the trend was also worsening in agricultural areas.
The report said the rate of agricultural land intensification in New Zealand has been one of the highest in the world, with dairy herds growing by 69 percent between 1994 and 2015, to 6.5 million animals - although sheep numbers were declining.
"Agriculture intensification does have an impact on water degradation ... what we can't say is which part of agriculture is having that impact. The simple answer of too many cows equals more nitrogen, we actually can't back that up with evidence in this report."
'We need to reduce the size ... of the dairy herd'
Environmental groups do not agree.
The Environmental Defence Society called today's report alarming. Its chief executive, Gary Taylor, said the report laid the problems with fresh water bare.
He said farmers should be required to obtain resource consents for agricultural land use in sensitive catchments, to set maximum levels of stock.
"It's all very well setting water quality limits in a national policy statement and in a regional plan ... but in the end I think we need to reduce the size particularly of the dairy herd in New Zealand by about a third.
"Particularly in these more sensitive catchments where we've got serious pollution."
Forest and Bird chief executive Kevin Hague said he wanted to know how many reports like this would be needed before the government took further action.
He said reducing stock numbers was an absolute bottom line.
"At the very least we need to be putting in place rules that mean farmers cannot intensify further ... so that we at least limit the inputs of these dangerous nutrients into rivers."
Regulation just one tool - Federated Farmers
Federated Farmers said it did not want more regulation on land use and farmers were already doing the work to help restore degraded water.
The organisation's water spokesperson, Chris Allen, said introducing tougher rules would not improve polluted waterways.
"It's more than stocking rate, it's actually going out there looking on the land, making sure that the practices we're doing have a positive - or understand if they do have a negative - impact on the environment, and farmers are very aware of that."
Mr Allen said regulation was just one tool and inroads would be made more quickly by farmers actually dealing with the issues themselves.
RNZ series Water Fools? - on air and online - looks at the troubled state of New Zealand's freshwater. Check out the full series here.