The union for ambulance workers is accusing Hato Hone St John and government ministers of being misleading and disingenuous over staffing levels.
Earlier this month, Hato Hone St John told RNZ the staff turnover was low and recent recruitment campaigns were successful.
Associate Minister of Health Casey Costello backed up the claim, and said the vacancy rate was the lowest it had been in a long time.
But First Union national ambulance coordinator Faye McCann said the service was struggling.
She said staff were burned out, and large numbers of them would leave if things did not change.
"The ambulance service is always the one at the bottom of the cliff. They're the ones stepping in when no one else is. To have that service not functioning properly is really concerning for New Zealanders."
She said it was difficult and "extremely frustrating" to hear claims there were no staffing problems when paramedics saw problems on the frontline every day.
"They're still the ones going to people who have been waiting too long for an ambulance."
She said Hato Hone St John officers frequently received text alerts about vacant shifts, due to low staffing levels in many regions.
McCann said that confirmed the service was struggling and the public should be very concerned.
Industrial action to call for more resources was now underway, she added.
It included chalking ambulances with messages and wearing strike badges - saying 'underfunded and undervalued' - instead of name badges.
But Hato Hone St John general manager for ambulance operations Stu Cockburn disputed McCann's claim.
He said it was not uncommon for staff to receive rostering messages offering additional shifts during the winter months.
"This is not a result of an overall resourcing or vacancy rate within the service, but because of high staff related illness or additional leave requests during this time.
"Recall shifts are offered per our collective employment agreement, meaning that fulltime personnel in many areas are offered these shifts first," he added.