Industrial action by radiographers over a wage freeze will vary from hospital to hospital, says the union representing the striking workers.
The Association of Professionals and Executive Employees last month instigated the action, which was due to end in most hospitals on Friday.
But the association's national secretary, Deborah Powell, says continuing action is the only option in view of district health boards' refusal to agree to any pay rise.
Ms Powell says some hospitals will be disrupted over the next three weeks by a complete withdrawal of labour for several days, and in other hospitals by radiographers working to rule.
At least 40 operations have been cancelled at Auckland City Hospital because of the strike.
The union says walk-outs are likely to spread to all of the 14 district health boards.
Only emergency X-rays are being carried out from Friday until Monday morning. MRI scans and CT scans have also been cancelled.
Auckland District Health Board deputy chief medical officer Margaret Wilsher says it's aiming to have the hospital occupancy rate down to 70% over the weekend by cancelling elective surgery and discharging patients where possible.
She says patients are heeding their requests to avoid using the emergency department where possible, although she says Starship hospital, where radiographers are also on strike, will be less affected.
Ms Powell says employers have refused to meet for negotiations and have made no pay rise offer despite an increase in productivity and a 3% budget rise for district health boards.
Hawke's Bay action extended
Meanwhile, at Hawke's Bay hospital, radiology staff have decided to extend their industrial action which was due to end on Friday.
The district health board says was told on Thursday night that the action will continue for another three weeks, and at times include a complete withdrawal of labour.
Over the past four weeks the hospital has had to reschedule a number of clinics and arrange for community health providers to help out.