Thousands of junior doctors are walking off the job around the country this morning for 48 hours.
Most doctors want to keep the working hours they have - which they have said are safer - but that district health boards want to change that.
It is the second 48-hour strike by junior doctors.
The doctors have voted in favour of a third strike that would take place on 12 and 13 February.
Hospitals are asking patients to call their GPs first if it is not an emergency during the strike.
Hawke's Bay District Health Board said the strike would affect some surgeries and outpatient appointments.
The DHB said it was well prepared for the action by members of the New Zealand Residents Doctors' Association.
Chief medical and dental officer Dr John Gommans said the priority was to continue to provide safe quality care during the strike period.
Hawke's Bay DHB employs about 153 resident medical officers [RMOs] and said many were union members.
Dr Gommans said essential and acute services would be prioritised during this time, which meant some outpatient appointments and planned surgeries would be rescheduled or put on standby.
Anyone affected will be contacted by the DHB.
"We will be re-booking those affected to the next available appointment, and we apologise in advance to those whose surgery or outpatient appointment is postponed," Dr Gommans said.
The emergency department at Hawke's Bay Hospital will remain open during the strike.
During the first strike earlier this month, junior doctors indicated they would not back down and called for DHBs to withdraw planned clawbacks to their terms and conditions.
Outside Wellington Regional Hospital up to 50 junior doctors were waving placards.
One of them, senior registrar Brian Corley, said DHBs were trying to remove some references to the doctors' union in their multi-employer contract.
He wanted to keep the Resident Doctor' Association included in the contract.
"We have elected them to represent us for a good reason. It's not up to the DHB bargaining team to tell us who represents us," he said.
"We will tell them who represents us ... the RDA represents us."