Junior hospital doctors are set to walk of the job for 24 hours next month, likely causing significant disruptions to services.
Their union, the Resident Doctors' Association, said it issued a strike notice to Te Whatu Ora - Health New Zealand on Monday after pay talks stalled.
Union secretary Deborah Powell said most of the 2500 doctors were offered a significant pay increase - up to 25 percent.
But they were striking for about 300 of their colleagues who were instead in line for a 12 percent pay cut, she said.
That minority group were training to be specialists in areas that had a big shortage of senior doctors - including psychiatry, radiation oncology and public health.
"We think Te Whatu Ora is bonkers, this is madness. Why on earth would you present a paycut to critical specialities?" Powell said.
A cut was also proposed for trainee general practitioners.
The junior doctors who were getting a pay rise did not want to see anyone left behind - particularly in vital services, Powell said.
"I'm really proud of them. They're making a stand to say 'No, it's not okay to have these registrars take a pay cut which will inevitably impact on the number of specialists we will have have in those areas. And, there are patients that need them and we have to stand up for them just as much'."
If the strike goes ahead as planned, some doctors would remain on the job to provide critical services, which would also be provided by senior doctors and non-union members, she said.
However, there would still likely be significant disruption for the strike period which will run for 24 hours from 7am on 7 May.
Mediation was scheduled for next week so there was hope the strike could be avoided, although Te Whatu Ora would have to step up, Powell said.
In a statement, Te Whatu Ora chief people officer Andrew Slater disputed the union's pay cut claim.
"In our initial negotiations we have guaranteed that no RMO [resident medical officer] will receive a pay cut," he said.
Te Whatu Ora planned to keep negotiating and hoped a strike could be avoided.
"Resident Medical Officers are a vital and valued part of the Health NZ workforce and we have already indicated that we are prepared to make a very significant investment in settling the ... collective agreement," Slater said.
Hospitals would start working on contingency plans in case the strike went ahead to ensure services could still be delivered, he said.