Frontline emergency crew are being replaced with less experienced staff so trained paramedics can cover private events, the union representing St John ambulance staff says.
About 1000 frontline staff have gone on strike and are refusing to come off the frontline to attend non-emergency private event work, which St John gets a private income stream for.
First Union said the ambulance workers were striking to get paid shift allowances, which would reflect the unsociable nature of the job.
It said St John was taking emergency staff from the frontline and transferring them to cover private sporting and commercial events.
First Union spokesperson Jared Abbott claims St John was then replacing these frontline staff with volunteers and casuals who are often in-training.
Mr Abbott said it was becoming more of a regular occurrence, "the events are actually getting preferential treatment over top of the public service which is what the government's paying for."
He said there was a lot of ill-feeling about this practice by the St John staff.
"People don't think it's right that tax-payers are essentially paying their wages to go and do private enterprise work."
Mr Abbott said the private events often required a paramedic to be in attendance and a lot or the volunteers and casual staff St John uses were not at that level.
"It's easier for them to take someone off the frontline and send them to the event then employ someone specifically for doing event work," he said.
St John receives 70 percent of its funding from the government, the organisation raises the remainder itself.
Mr Abbott said St John do not advocate to be a fully funded government service, "they actually like only getting 70 percent funding so that they can really have control over being able to do this private enterprise work."
The union said the industrial action was not putting the public at risk because other ambulance services would be paid to cover the private events.
Mr Abbott said ambulance staff work a rotating shift that was not regular, four-on-four-off rotate, which covered overnights and weekends and averages out at 42.5 hours a week and were not rewarded for that work.
He said every other service in the District Health Boards receive some kind of allowance for working night and weekend shifts, except ambulance staff.
Mr Abbott said the wage rates across the ambulance service were low and a qualified ambulance officer would get around $54,000 for the longer week.
St John said it was in the middle of collective bargaining with its four unions and hopes to reach an agreement that suits all parties.
It did not respond to questions about frontline coverage being carried out by less experienced staff.
The partial strike will continue until a collective settlement is agreed.