Proposed national water quality standards are weak and will allow for further degradation of New Zealand's rivers and streams even to levels seen downstream of raw sewage outlets, the Green Party and water scientists have warned.
The ministry is working on a "National Objectives Framework" that sets maximum levels for key pollutants, slime and harmful bacteria in waterways.
But a draft version leaked to the Weekend Herald was criticised by Massey University's Russel Death as having "extremely weak" limits on key measures, particularly those for nitrates in rivers and streams.
The draft document sets limits based on toxicity or the level at which high concentrations of nitrates - typically from fertiliser run-off - poison fish and other animals.
However, the bigger issue with nitrates and other forms of nitrogen was that they stimulated growth of periphytons, the slimy algae that forms on river beds and changes the water chemistry.
"The nitrate values you'd be looking for to prevent those kind of things are an order of magnitude less than what they've got for toxicity," Dr Death said.
The maximum nitrate concentration proposed in the document is 6.9mg a litre of water, which Dr Death said would result in "out of control" algae growth.
"Once they get over about 1.5mg you're really starting to get into some quite excessive periphyton growth."
The proposal for a maximum permissible level for periphytons was rarely seen in New Zealand, said Dr Death, who'd only heard of similar readings "downstream of very poorly treated sewage coming into waterways".
Other water scientists the Herald spoke to were unwilling to comment on the draft standards until they were publicly available but one questioned the value of setting minimum standards that allowed for concentrations of pollutants that were rarely seen in New Zealand anyway.
"How helpful is that because most of our water bodies aren't getting anywhere near that so it's almost as if you're setting a bottom line that you waltz home through."
Green Party water spokeswoman Eugenie Sage said the draft objectives were not only weak, but entirely omitted important measures of the health of rivers, lakes and streams including invertebrate and sediment levels.
The weak safeguards for nitrates and other chemicals were "virtually a licence to pollute".
"What they're effectively saying is we can allow the rivers to degrade and algal blooms to become the norm."
Ministry for the Environment policy deputy secretary Guy Beatson said no decisions had been made over the objectives, the draft framework was incomplete and it had not been seen or approved by ministers.
Environmental Defence Society policy director Raewyn Peart said the objectives, which were to be the keystone of New Zealand's freshwater management regime, seemed "weak and incomplete".
"The draft will need significant tightening if it is to provide an effective framework and environmental bottom lines for regional councils to work under."