A new bill aims to tighten controls on sustained pollution of waterways by closing a legal loophole.
Green MP Catherine Delahunty said her member's bill, which has passed its first reading, sought to close a loophole in the Resource Management Act that allowed contaminating discharges with toxic effects and discolouration of waters under "exceptional circumstances".
Ms Delahunty said the phrase included no timeframe, and had been used to justify long-term pollution of some waterways and coastal areas.
Her bill would limit its use to five years.
Ms Delahunty said the bill would apply only to those "worst polluters" which discharged waste that would seriously discolour water and have significant toxic effects.
"Changing the law to define exceptional circumstances as five years only is an important step in protecting waterways and the coastal areas so it cannot be used to justify ongoing pollution by councils and industries as it has been used in the past."
She said the most "legendary example" was the Bay of Plenty river that inspired the name and purpose of her "Black Drain Bill".
The Tarawera River, the subject of decades-long arguments between iwi and Kawerau mills blamed for the pollution, was listed as a no-go zone at its rivermouth beach near Matata in the Ministry for the Environment's new report card on water quality.
"Locally it's important to change the law and provide an incentive to the pulp mills to work towards a clean production process that doesn't use the river as a drain for pulp mill wastewater which contains resin acids and bleaching chemicals," Ms Delahunty said.
"The river people, three iwi and local residents would love to see the river restored, as upstream of the mills it is a pristine river, for fishing and recreation as well as cultural identity."
Bay of Plenty Regional Council member Tipene Marr, who has been battling to restore the river for decades, said his father had hoped to see the river clean again in his lifetime.
"He died in 2000, but a lot of the old-timers who live in the area would love to see the day when the river runs like it does south of Kawerau - lovely and clean, where you can drink it."
Under such an "exceptional circumstance", an Environment Court ruling allowed the Tasman Pulp and Paper mill to continue discharging effluent into the river for a further 25 years.
While historical bleaching processes had produced high level of organochlorines, newer technology used at the mill produced very low levels, said Bay of Plenty Regional Council's environmental management general manager, Eddie Grogan.
He said the last challenge was the reduction of colour discharge.
Health effects from swimming in fouled rivers and swallowing water tainted with faecal micro-organisms or other bacteria are largely minor and seldom life-threatening, but can be unpleasant.
Symptoms could include diarrhoea, vomiting, and infections of the eye, ear, nose or throat, while children were particularly at risk of ear and skin infections, said Dr Phil Shoemack, medical officer of health with the Toi Te Ora Public Health Service.
Such conditions usually lasted two or three days.
"When you add up all the number of people who are potentially exposed - all the children who might swim in the water or all the people camping whose summer holiday might be at least altered - the impact adds up."
The Environment Ministry also warns of more harmful diseases such as giardiasis, cryptosporidiosis, campylobacteriosis, salmonellosis and Hepatitis A.
Dr Shoemack advised people to avoid suspect waterways and not swim for two days after rain.