Ambulance workers have been protesting work conditions and pay. Photo: Supplied / First Union
First Union ambulance officers who work for Hato Hone St John will walk off the job for the first six hours of their shifts on Friday.
The industrial action will last for 24 hours, starting at 4am.
They are also planning to strike on Monday over pay and work conditions.
First Union spokesperson Faye McCann said ambulance services were too important to rely on charity funding.
She was disappointed that another union, the Ambulance Association, has ratified St John's latest pay offer.
Striking St John ambulance officers will be collecting signatures on Friday for a petition calling on the government to fully fund ambulance services.
Stu Cockburn, Hato Hone St John general manager ambulance operations, said in a statement on Wednesday the service's top priority was always the safety and wellbeing of patients.
"We are working to ensure that our contingency plans will effectively meet patient needs during the strike."
Hato Hone St John expected the impact to be primarily in the Auckland region, and some parts of Waikato and the Bay of Plenty.
"We have been working to ensure we can continue to respond to patients with life-threatening and time-critical conditions and remain committed to keeping the public and our stakeholders informed as we navigate this next round of industrial action.
"As the New Zealand Ambulance Association division of the Amalgamated Workers' Union New Zealand - the larger of our two unions - has accepted the Hato Hone St John pay offer and is not participating in strike action, we expect this strike to cause less disruption for patients than the strikes that took place last month."