District health boards in Auckland say other staff have coped well during Friday's strike by about 400 medical laboratory workers over an unsettled pay claim.
The lab workers escalated weeks of low-level industrial action to a full walkout at Auckland, Counties Manukau and Waitemata.
They walked off the job at 1am and will stay out until midnight.
The secretary-treasurer of the Medical Laboratory Workers Union, Bryan Raill, says the strike is a protest against punitive suspensions and a lack of progress over pay.
A DHB spokesperson says staff have managed well in all three areas, though the strike has had a significant impact on patient waiting times.
The union called off all strike action in Canterbury because of the earthquake.
63 non-union lab workers on duty
The Auckland DHB says it has 63 non-union lab workers on duty on Friday and is confident of coping.
Counties Manukau and Waitemata DHBs, with fewer non-union workers available, are urging the public not to visit the hospital emergency departments if possible.
Arrangements are in place at all hospitals for emergencies.