Anaesthetic technicians at Hawke's Bay District Health Board are going on a 48-hour strike.
The APEX union members are pushing for better pay, and it is their third strike in several months.
National secretary Deborah Powell says all eyes are on the DHB.
"How many we train is up to the district health boards, and unfortunately they haven't been training enough," she told Newshub. "That means we've got a shortage. We just don't want to lose the advantage in Hawke's Bay that we have, that has seen us full staffed."
APEX union workers will strike until Wednesday morning.
"If you don't have anaesthetic technicians, then surgery can't go ahead," said Dr Powell. "Emergencies will still go ahead… but electives will be postponed and cancelled."
The DHB says 38 elective surgeries have been put on standby or postponed.
"Anaesthetic technicians are an integral part of operating theatre teams and any operation, requiring a general anaesthetic, must have a technician present," a spokesperson said.
Ms Powell says this is short-term pain for long-term gain.
"What we're trying to do is stay fully staffed so we can continue to deliver a really good service to our patients. The district health board is trying to drag our pay down… which we know will result in recruitment and retention problems."
Apex and the DHB failed to come to an agreement during talks last week. Negotiations have been taking place since July, when the previous contracts expired.