The New Zealand Professional Firefighters Union said a power converter ignited and burned out in the cab of the vehicle as it returned from a factory fire.
It said it triggered an evacuation and crew to get fire extinguishers at the ready.
Fire and Emergency said no fire was involved.
The union's Tauranga branch secretary Mike Swanson told Morning Report elderly trucks were being used more than ever.
"They are there to replace frontline appliances as they go to service but they never seem to get a rest themselves because they've been pushed around from station to station."
Swanson said there were new trucks on the way but they would take some time to arrive.
"Till the new ones come in, we don't get another truck."
In a statement, FENZ said the truck had an electrical issue which meant it gave off a burning smell.
"Our firefighters and support staff reacted promptly, and the charger was replaced within an hour."
The spokesperson said an alternative truck was available and still is, should an urgent issue arise.
They said they had a replacement programme and that 70 new trucks were on order.
"Since 2017 around 317 new trucks have been added to the Fire and Emergency fleet."
