St John staff are offering free ambulances until they win their battle over shift pay.
More than 1000 FIRST Union workers want St John to come to the table and make changes.
Organiser Sarah Stone says workers have been in bargaining since September.
"We've got a mediation date in the middle of January, but the strike action will continue until our bargaining is concluded and our members have an offer they can accept."
She says workers have had enough.
"They can't continue like this. We're asking the Government as well as St John to put aside money that's just going to go into those salaries to meet those needs."
- Stolen St John ambulance found in South Island
- St John ambulance workers to strike over pay, staff misuse
Ms Stone says shift recognition payments are a basic requirement for any 24/7 worker.
"While we're sleeping at night, they're out saving lives. While we're spending time with our families at weekends, they're out there working and saving lives - and they're not recognised for that in their salaries at all."
Patients remain the priority, the union says. Ms Stone says the action is targeted at St John and won't affect the safety of passengers.
St John said it respects its staff's "right to take industrial action while working in a safe and professional manner", but patients will still need to cough up.
"While there may be a delay in receiving an invoice during the holiday shutdown period and FIRST Union industrial action, we will continue to invoice patients for the part charge."