Hospitals were making plans already, cutting services and taking out radio and newspaper ads telling people how they will be affected.
DHBs' national contingency planner Anne Aitcheson said hospitals would normally try to reduce their occupancy to about 85 per cent during a strike.
But that had not been possible because winter illnesses such as RSV meant there were a lot of people who needed urgent care, she said.
That meant they had to ask more nurses than usual to stay on the job for essential care.
Both sides have to agree on the number of union staff who need to stay working to provide what are known as life preserving services.
The NZNO industrial services manager Glenda Alexander said it had been difficult to get enough to put their hands up to work during the strike in some big city district health boards, and in some mental health services.
That was because they were frustrated with chronic understaffing, she said.
"I guess they are saying - we have to put up with these unsafe conditions all the time so why should we go further to help out when we are in a dispute."
They now had enough for the strike, she said.
But Aitcheson said the nurses were not being asked to cover above and beyond normal staffing levels - and final details for Thursday were still being sorted.
Emergency services would still be available for those who needed them, she said.
Earlier this month, the DHBs took the Nurses Organisation to the Employment Court over the details of how the staffing agreements are worked out.
A judge has reserved his decision.
