Photo: ST JOHN NZ
St John Ambulance said it was having a busy night and asked people to reserve 111 calls for life-threatening emergencies only.
About 2500 ambulance officers were on strike for the first four hours from the beginning of their shifts.
The action started at 4am for 24 hours.
A second strike was scheduled from 4am on Saturday.
St John and the unions have been bargaining over pay and conditions since since their collective agreement expired at the end of last year.
Dan Ohs, Hato Hone St John deputy chief executive Emergency Ambulance Operations, said tonight the ambulance service is experiencing an increase in demand and would ask people to reserve 111 calls for life-threatening emergencies only.
"We would remind the public that during strike action they can still call 111 and get help in a life-threatening emergency. For all non-emergencies we are asking people to please remember to contact Healthline or consider self-transporting to an urgent care clinic or hospital," Ohs said.
"Fortunately Hato Hone St John has not identified any adverse incidents or patient harm resulting from strikes throughout the day, and are focused on safeguarding patients through to tomorrow morning."