Partially untreated drinking water had to be swiftly diverted into the Wellington region's wastewater network after a power cut stopped a UV filter from working.
Wellington Water reported the incident, which happened in November last year, to the Ministry of Health.
But Wellington City councillors have only just found out about it today at a City Strategy and Policy Committee meeting, leaving some of them fuming.
The incident is briefly referenced in the council's Quarter 2 report as a forecast non-compliance with Drinking Water Standards due to a process failure at the Waterloo treatment plant.
Councillor Iona Pannett raised the issue this afternoon.
"It looks like we're failing on a very critical metric which was going okay."
Chief infrastructure officer Tom Williams was called in for a "please explain". He said a power outage caused the system to trip and the UV treatment to completely cut out.
Greater Wellington Regional Council owns the Waterloo Water Treatment Plant, which supplies water to Wellington City.
Water is treated at the plant with a "double barrier" protection system where it is firstly chlorinated and then filtered with UV to remove impurities.
The situation triggered an alarm and was immediately dealt with by discharging any water into the wastewater system that had not undergone UV protection.
"At no point has any of that water gone into the network to residents or ratepayers", Williams said.
Williams noted that while the operational response was successful, the manner in which councillors were notified was not.
He said he only found out about the event when he read the quarterly report.
Wellington Water has been approached for comment.
Councillor Fleur Fitzsimons said she was surprised to only be finding out about the incident, which she characterised as serious, today.
"That's unusual and alarming that we don't know."
Fitzsimons asked Williams whether Wellington Water was treating the council with the respect it deserved given their fundamental role in the company.
"This doesn't feel like it", she said.
Williams, who has been in the role for about six months, said the quarterly report raised governance and oversight issues he could learn from.
"Are they [Wellington Water] treating me and you like a 100-million-dollar client?", Williams said addressing Fitzsimons' question.
"We've got an improvement plan in place", he said.
An independent report was commissioned last year into the relationship between Wellington Water and Wellington City Council.
It found it was lacking trust and in need of a "fundamental reset".
"There are difficulties and niggles occurring at each level and every part of the relationship between the two organisations", the report said.
Following the findings, the council and Wellington Water have regular meetings at a chief executive and senior executive level.
The council has also created the Chief Infrastructure Officer role, who is a member of the Executive Leadership Team and is responsible for the working relationship with Wellington Water.
The council has established a special office to undertake further work on Strategic Asset Management.