Firefighters' emergency breathing air-tank filling equipment has been contaminated with asbestos.
The discovery has shut down air filling at Auckland City fire station, and it has withdrawn all its breathing apparatus tanks.
The shutdown is causing "limited delays" in responding to calls, Fire and Emergency (FENZ) told RNZ.
The problem is contamination found of an air intake beside an asbestos sheet roof.
Initially, FENZ told RNZ the contaminated air intake was for an "historical compressor system which is no longer in use".
However, it later said the intake was used from 2015 and up until last year.
A new system installed last year had tested negative for asbestos but had also been shut down as a precaution, FENZ said.
The system - the compressor plus storage tanks, pipework, filtration systems and a filling panel at Auckland City Station - is locked down and breathing apparatus cylinders are being supplied from other station stocks.
Firefighters told RNZ the contaminated air intake had filled hundreds, if not thousands, of tanks from a wide area, for FENZ and other emergency services, including police.
But FENZ said: "It is not common practice for Auckland City Station to fill BA cylinders for other agencies."
Police said they were "not immediately aware" of an issue.
Initially, FENZ said both the old and new air-filling systems had a "sequence" of filters.
When firefighters questioned if these were designed to block asbestos, FENZ then told RNZ it was "standard" filtration, and "we are working with the manufacturer to confirm the precise level of filtration provided".
"It is ludicrous to be bottling air next to an asbestos roof," said one firefighter, who RNZ agreed not to name.
"It is extremely concerning that we have possibly introduced asbestos into our breathing air."
The Professional Firefighters' Union said it was "appalled".
The air intake discovery underlined that for years firefighters and staff had been "repeatedly exposed to asbestos" and indicated wider "systemic failures", the union said.
The compressor used up until 2022 was used to fill bulk storage tanks.
"More testing of the current BA compressor system and a sample of cylinders is being undertaken," FENZ organisational strategy and capability development deputy chief executive Sarah Sinclair said.
The previous compressor was put in storage last June pending transfer to a planned FENZ facility at Whenuapai, Sinclair said.
"It is currently still in storage and has not been used since," she said.
"It will undergo the same rigorous testing as is underway for the current compressor system at Auckland City Station before it is put into use."
Auckland district has seven other filling stations and one mobile filling station, and is now relying mostly on Mt Wellington and Avondale.
Auckland station's new compressor was not connected to the historical intake pipes and was in a "dedicated and controlled room", Sinclair said.
"We take our people's safety and wellbeing extremely seriously," she said in a statement.
"We are confirming with our equipment suppliers options to replace the compressor air intake filters and will conduct additional testing."
There was some short-term disruption for Auckland City station, but it was still able to meet response requirements across the city with "limited delays".
A firefighter said: "I'd like to think that this will prompt WorkSafe to do some actual proactive investigation for themselves."
WorkSafe has said little publicly about the contamination, except to stress the onus was on FENZ.
It took two weeks for WorkSafe to issue a statement in response to RNZ's initial inquiry in April.
On Monday, it said it was gathering information to "determine if there are any issues the regulator needs to take enforcement action on".