Public hospitals say they are well prepared for the three-day strike by several thousand junior doctors.
Members of the Resident Doctors Association throughout the country went on strike at 7am.
At the heart of the dispute are rosters involving as many as 12 consecutive days at work, which the doctors say are unsafe.
District health boards say they have agreed to fix the rosters, but can't agree to the union having the final say over roster changes at individual DHBs.
That's a key issue in the talks, which led to a 48-hour strike in October involving all 20 DHBs.
This time all DHBs are involved except Taranaki and West Coast - where DHBs say there wasn't enough support for another strike.
Hospitals have been postponing and rescheduling outpatient appointments and non-urgent surgery, and say they're ready for the strike.
But the national contingency planner for DHBs, Anne Aitcheson, said people should not delay seeking care if they are unwell.
"Always, if you are unwell, seek medical help - don't delay because there is industrial action.
"Go to either your general practitioner or your hospital, read the advice of your local DHB and ensure you get the medical care you need."
The strike will last 73 hours, ending early Friday morning.