The professional firefighters' union is warning there will not be enough crews left for callouts when the Covid vaccine mandate kicks in next week.
Fire and Emergency is requiring 13,000 staff or volunteers to be vaccinated to do their jobs after the 15 November deadline.
The union estimates this may keep 10 percent off the frontline, from car crashes as well as fires.
"This is the crisis as of midnight on the 15th," the union's national secretary Wattie Watson said.
"Fenz, in our view, will not be able to maintain the current level of response due to the number of firefighters, both career and volunteer, that are either unvaccinated or their vaccination status is not known."
Crews would be hobbled if they lose one or two key staff. They had already been struggling to fill rosters, especially in Auckland and Wellington, Watson said.
The government is being asked for a temporary exception to the mandate to sort this out, and the union believes Fenz had asked the same.
In a statement this morning, Fire and Emergency national commander, Kerry Gregory, said the organisation was in the process of making sure the organisation complied with the Covid-19 Public Health Response (Vaccinations) Order.
"We are securely collecting vaccination status information to comply with the Order and assessing the implications of this mandate for our people, including employment matters, and on the contingency planning we've had in place throughout the Covid pandemic," Gregory said.
"We always have contingency plans in place so we can continue to serve our communities across the country.
We are working together with our Unions and Associations, and we appreciate the support they are providing our people... We will continue to keep our people, and our Minister, updated as we work through all implications of this mandate. We are looking at all options available to ensure we reach a satisfactory outcome for our people and our communities."
Firefighters and volunteers had a deadline of 5 November to tell Fenz their vaccination status.
Watson said the government had been late confirming only on 28 October that firefighters were covered by the mandate, and Fenz had not planned or prepared properly for it.
Some of their 2000 members were stressed by the threat of losing their vocation, she added.
Their first meeting about the possibility of no-jab-no-job was only this Monday, and at that Fenz raised the proposal of putting staff who could not do their role on unpaid leave, she said.
"We told them that is untenable, unlawful and inhumane."