Lives are at risk over a decision by Fire and Emergency New Zealand to remove high-ladder trucks from initial responses to Auckland emergencies, the union says.
But FENZ has denied the public are in danger, saying a heavy aerial appliance - which has ladders up to 30m - is stationed just five minutes from the central business district if needed.
It was the delayed arrival of such a fire truck at the Grenfell Tower inferno in London that contributed to the tragedy, in which 72 people died in June 2017, one veteran says.
New Zealand Professional Firefighters Union Auckland local secretary Martin Campbell, a firefighter for almost 25 years, said FENZ's order to remove heavy aerial trucks from a first response could result in the same large-scale tragedy in Auckland.
"The risks and dangers of aluminium composite panels [ACP], the major contributing factor to the Grenfell Tower tragedy, are still unknown," Campbell said in a notice to Auckland union members.
"We consider a fire alarm activation as a fire until proven otherwise."
Auckland Council found 116 buildings that appeared to use ACP, and 25 that used it in exterior cladding like at Grenfell Tower, including some Auckland Hospital buildings and large residential blocks in the Viaduct and CBD.
The rapid spread of the Grenfell Tower fire up the exterior of the 24-storey residential block was blamed on ACP cladding.
A fire truck such as Auckland's two heavy aerial appliances with turntable ladders or a cherry picker that extend 30 metres, or 10 storeys, did not arrive for 32 minutes by which time the fire was out of control.
An uncontained fire doubled in size every two minutes, Campbell said.
"They're putting people's lives at risk and also our firefighters' lives at risk because it's us that have to try and come up with effective strategies and tactics to work around the fact we haven't got the proper equipment available to us that we should have."
Before the reduction, four appliances turned out to Auckland emergencies: three standard trucks and a heavy aerial, carrying a total 16 firefighters.
"It's the safe minimum number of firefighters and fire trucks to enable us to begin a fire attack or a rescue."
The FENZ order has reduced that to 12 firefighters, though Wellington and Christchurch are not subject to the directive.
Campbell believes there are several factors at play.
One was that FENZ could no longer recoup the cost of false alarms, of which there were more than 26,000 in a year.
The callouts cost FENZ millions and since July 2017, when urban and rural fire services joined forces to create FENZ, businesses that have more than two false alarms in 12 months are no longer fined $1000 plus GST - a cost now funded by the fire service.
The other reason was the state of the country's ageing fire truck fleet.
One of Auckland's two heavy aerial fire trucks has broken down and the city currently relies on a second at Parnell Station and a North Island relief aerial truck usually shared between Hamilton and Auckland.
That relief truck, estimated to cost $1.2 million, is also out of service.
It's in stark contrast to when Auckland had four heavy aerial trucks in the 1980s, serving a much smaller population.
NZPFU national vice president Joseph Stanley said the country's entire aerial fleet had significant faults.
"Every one of the metropolitan cities in New Zealand is affected by a faulty aerial appliance."
The situation was the result of "tremendous" under-resourcing and a "severe lack of strategic vision" by FENZ into how to maintain the fleet capability, he said.
"It's terrifying for firefighters. It's not acceptable for the capability to be lessened at any time, especially when our communities need us the most."
FENZ region manager Ron Devlin said the decision last year to remove aerial trucks from initial callouts came after the trucks responded to 2520 fire alarms in the past five years, but were only used at seven.
Every one of the metropolitan cities in New Zealand is affected by a faulty aerial appliance.
The decision was made to reduce wear and tear on the appliances and ensure they were available when needed, Devlin said.
"... they remain available to attend other calls for fire safety education, other risk reduction activities in the community and for training purposes."
The change did not increase public risk, he said.
"If the officer in charge at the incident decides that an aerial appliance is needed, an aerial appliance is dispatched."
FENZ was reviewing its aerial appliance strategy, Devlin said, and had started a project to purchase new aerial trucks.
"The public can be assured that our fleet is well maintained, safe and fit-for-purpose.
"From time to time, as with all vehicle fleets, our appliances require maintenance. When appliances require maintenance they are repaired promptly and replacement appliances are provided in the interim."