Auckland Local secretary John Waldow said fire communications centre staff were not adequately trained or supported to give CPR instructions in an emergency call.
"They would have been having to do medical work essentially. These staff have received zero training on this."
He said to make matters worse most of the fire communications centre staff did not have a current workplace first aid certificate because they had expired.
"So now you have somebody who isn't up to date in first aid going to give potentially lifesaving instructions over the phone."
The union's biggest concern was the lack of support if something went wrong.
"The control room operator is a human being as well. They need to be supported and trained to do this if this is how it's going to go."
A FENZ spokeswoman said the directive was almost immediately rescinded.
The CPR instructions were "intended to help those staff deal with calls that needed immediate CPR help", she said.
"Our normal practice is for our operators to record people's details and transfer them to the ambulance so the advice was recalled quite quickly and that normal practice continues."
New Zealand Faculty Chair of the Australasian College for Emergency Medicine, Dr John Bonning, said every minute counted in emergencies such as a cardiac arrest and it was imperative callers were provided with advice that could save lives.
"If somebody can start CPR within 30 seconds or a minute their life might be saved. There are absolutely lives saved by the immediacy of response and advice to do bystander CPR."
Defibrillators were also crucial but Bonning, who is clinical director of the emergency department at Waikato Hospital, stressed that 111 calls must be for emergencies only.
"We do need to unburden emergency lines because lives will be saved by immediacy and response in those real emergencies, particularly in CPR."
Medical calls to fire staff were increasing according to a staff member, because of an automated answer system that kicked in when Spark 111 call takers were overloaded.
A limited number of Spark 111 call takers across the country forward emergency calls to either police, fire or ambulance.
But when the calls begin to queue it triggers an automated system, known as Interactive Voice Response (IVR), where the calls are diverted and the caller selects an emergency service from a pre-recorded message.
When the IVR is triggered it remains active for at least 30 minutes and up to an hour over the New Year. It was activated on December 22 and is currently in place until February 1.
Meanwhile police 111 calls were also spilling over to fire call takers, with one communications staff member receiving a call from a man "getting a beating on his front lawn".
Last month police said 111 calls to them exceeded 77,000 in November, the most of any month last year not including December.
Communication Centres national manager Superintendent Dave Trappitt said there was increased pressure on call centres and with longer than usual wait times possible.
Police were recruiting new staff and seconding staff from other areas to cope with the increased demand.
