A Christchurch city councillor wants the local government minister to help reverse a decision to chlorinate the city's drinking water.
Taumata Arowai, the new water regulator, recently mandated chlorination in response to 2016's deadly camplyobacter outbreak in Havelock North.
It declined an application for Ōtautahi to be exempt - saying the city was failing to reach basic standards, let alone those for an exemption.
The main supply, sourced from underground aquifers, had been largely chlorine free apart from a few temporary stints following the Canterbury earthquakes in 2011.
In its draft decision, Taumata Arowai declined two bids made by the council last year to be exempt from the mandate.
It said the risk management plans were not specific enough to the water zone in the application, it did not identify risks at an individual bore level and some upgrades to chlorine infrastructure and bore heads was yet to occur.
There was also unacceptable water loss in the system, not enough detail about historical contamination events and not enough backflow measures.
Taumata Arowai also said the supply had no primary barrier, such as UV treatment.
Christchurch water quality will vary across city's 156 wells - scientist
Groundwater scientist Louise Weaver said there were natural filters in Christchurch's underground aquifer system.
Atomic charges could repel contaminants, microrganisms could eat them and the water could physically move particles out of its flow.
But she said the water quality will vary across the 156 wells that supplied the city.
"The length of time that the water remains in those aquifers can be a risk factor because you can't rely on that process to begin immediately," Weaver said.
"If you think back to things like Havelock North... that water source was quite fast flowing so there wasn't the time for the contaminants to be removed."
Christchurch's wells were drilled to different depths, so some water had spent more time being naturally filtered than in other wells.
Installing UV and chlorine treatment infrastructure at each pump station would help the city's case, Taumata Arowai noted.
But the cost was likely to be in the hundreds of millions because Christchurch had 53 stations.
The council's acting head of Three Waters, Brent Smith, said the council was aware there was a lot of work ahead, even though an extensive programme to improve its bore heads had already been running for years.
"The draft decision has given us more clarity on how the new regulations are interpreted and applied and now we know that if we want to successfully gain exemptions for any of our water supply zones the level of detail and evidence we'll need to provide," he said.
"The bar that needs to be met, is high.
"A lot of work to make further upgrades to our water supply network is already underway or in the pipeline. Additional work is now being scoped by the Three Waters team and will be brought to the council in the future for consideration."
Plea for central government to intervene
Councillor Sam MacDonald wanted central government to intervene.
"The government have given a significant amount of power to an independent regulator, which means that effectively you have unelected officials with a significant amount of influence over people's everyday lives," he said.
"I think when situations like this arise, the government should get involved."
His petition had already received more than 3400 signatures in five days.
MacDonald believed Taumata Arowai was being over the top in its application of the standards.
"Our view [as a council] was always that as long as we could demonstrate risk mitigation, we would be at a point where we could have chlorine removed," he said.
"People have not died from our water, they don't get sick from our water. We have a very good water supply, that people can have confidence in.
"The only thing that's changed is rules, not the risk to people," MacDonald said.
"What we really need from the regulator is clarity on, effectively, how much more we need to spend to achieve that exemption.
"The biggest challenge we have is that [Taumata Arowai] can't tell us what that [figure] is because they're effectively saying the bar is set so high that we'll never be able to achieve it," he said.
Christchurch City Council had invited Taumata Arowai's chief executive to explain the decision at a public council meeting next month.