Around 150 surgeries have been cancelled in Rotorua, as anaesthetic technicians walk off the job on Monday.
The seven-day strike comes after failed negotiations with the Lakes District Health Board (DHB) for improved rest breaks.
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Union APEX has been bargaining with the DHB for around a year-and-a-half. It's calling for the reinstatement of a mandatory 12-hour break between shifts.
"The provision for a 12-hour rest break when 10 hours or more had been worked had been in place for decades - before the DHB stopped the practice," APEX said in a statement.
"Agreement was reached in bargaining to reinstate the 12-hour break, however the DHB rescinded its agreement in December."
But the DHB is questioning why Rotorua's technicians are protesting over the break when other regions didn't.
"Given that anaesthetic technicians under the APEX and PSA unions have settled at other DHBs, without raising the issue of a 12-hour break, I have to ask the question, why is the APEX union and its Rotorua members targeting Lakes DHB?" acting Lakes District DHB CEO Nick Saville-Wood said.
He said the DHB's 11 union member technicians walking off the job could have great consequences.
"Lakes DHB is becoming increasingly concerned about our community being able to access hospital services as our priority is to ensure patient safety and wellbeing is maintained."
APEX said technicians will still be available for life-preserving acute surgery where patients cannot be transported to other providers or hospitals.
The strike will end at 8am on April 15.