St John has been accused of trying to dock the wages of ambulance officers taking part in strike action, according to their union.

Ambulance officers are now in their third month of industrial action after collective agreement negotiations over pay increases, stress levels, and work hours broke down.

The action requires paramedics to wear t-shirts that read 'Healthy Ambos Save Lives', and includes a ban on attending private events over the summer. However, officers will still attend all emergency calls.

In response, First Union claims St John is threatening to deduct ten percent from ambulance officers' wages if they don't wear hi-vis vests over the union t-shirt, citing health and safety rules.

Jared Abbott, First Union spokesperson, believes St John's actions "disgraceful."

"This type of action costs no money to the company," he said. "This action was specifically taken in order to not cause a disruption to the company or to the public."

"[St John] don't make people wear hi-vis vests when they're in their uniform so the idea that an ambulance officer would need to need to wear a hi-vis vest while they're in the actual ambulance for their own safety is ridiculous," he said.

Last month, paramedics told Newstalk ZB that under-staffing and single-crewing in certain areas was leading to patients dying "on a regular basis".

"Your eyeballs are hanging out and you know it's all about chasing numbers not about getting the right care to people," one officer said on condition of anonymity. "That's really frustrating and demoralising."

Abbott said more action is likely after St John's decision, and their view of the company has hardened.

"Members are asking to ballot for industrial action that will have much more impact on the company," he said.

"We're working through how we do that because the last thing we want to do is put the service at risk."

"It's already stretched beyond its limit."