Auckland Council is being taken to court over a consent allowing it to discharge stormwater into the environment over the next three decades.
But the council says that runoff of stormwater would happen whether a consent was in place or not.
The council recently approved a resource consent that would allow its Healthy Waters unit to divert and discharge stormwater across the region for the next 35 years.
The city's stormwater network channels rainwater from private properties, public reserves and roads through some 6000km of pipes and culverts, many of which divert stormwater into streams, rivers and the ocean.
In the past, these diversions were authorised by a patchwork of authorisations and consents inherited from seven former councils.
The new Auckland Stormwater Network Discharge Consent (NDC) replacing it would bring all of these into a single set of requirements.
While the consent referenced some strategies and targets to reduce the negative effects that came with stormwater runoff, Forest & Bird has appealed it, arguing it lacked clear rules that would stop streams and the coastal environment from being damaged.
Stormwater could contain heavy metals and other contaminants that pollute streams and the sea, the group's Auckland regional manager Nick Beveridge said, while mud carried out could wash into streams and bays, coating the sea floor with sediment and suffocating sea life.
"The council has basically given itself a blanket consent to discharge stormwater from its existing stormwater networks and any new networks it might develop within the next 35 years," Beveridge said.
"This consent is a serious concern because it doesn't set clear requirements to make sure the council does not damage waterways, the marine environment or native fish and sea life."
Beveridge was concerned the council's consent covered future development in the next 35 years, without adequate environmental protection.
"Auckland has some important ecological areas with rare birds and amazing sea life and we don't know where the new stormwater discharges will go," he said.
"There could be stormwater discharges from new developments, such as big subdivisions, and we're worried about the impacts.
"There are not enough conditions in the council's consent to be sure the coastal environment and significant ecological areas won't be harmed."
The worries come after years of outrage about pollution at Auckland's beaches, with many popular spots closed over summer due to contamination.
Healthy Waters general manager Craig Mcilroy said stormwater occurred every time it rained and water landed on a hard or impervious surface.
"The runoff of stormwater overland or through the piped network into streams, estuaries and the sea would happen regardless if Healthy Waters held a consent or not."
He said a regionwide consent would mean savings, efficiency, consistency and improved environmental controls.
"All discharges will be covered by a consent that reflects best practice in stormwater management," he said.
"Currently, there are some parts of the city operating under a temporary authorisation."
The consent had a triennial performance and six yearly review requirement, he said, so the council could evaluate how well the stormwater network was performing against the outcomes and targets and refine how stormwater was managed in response.
The council was undertaking hundreds of millions of dollars of stormwater upgrades, including a $361 million programme to improve beach water quality in the Waitematā harbour and to help reduce volumes of wastewater going to the Manukau Harbour.
Another $54m project in the council's 10-year Budget aims to reduce the amount of litter and road pollutants flowing into drains.