Patients requiring medical help were likely be greeted by ambulance officers in mufti today after mediation talks failed this week.
Nearly 1000 of First Union's St John members in the North Island wore union badges and went casual to protest working hours without putting any lives at risk.
Spokeswoman Lynette Blacklaws said St John had threatened disciplinary measures against staff. She said the threats were "good" because it meant St John was listening. But she was also disappointed.
"It's quite appalling that St John is behaving like this. All their staff are doing is taking low-level action, we're not putting anyone's safety in jeopardy."
Blacklaws told NZME the threat of disciplinary action had made protesters more passionate, "given them that extra oomph to carry on".
She said the union wanted St John to demand more government funding so ambulance staff did not have to work 14-hour shifts without a break.
Blacklaws said it reflected issues being heard elsewhere in the health sector, citing junior doctors who were complaining of long hours with few breaks because of a lack of funding for district health boards.
"They [ambulance officers] aren't asking for much and their hands are tied as to how much they can do. The uniform ban is just a way to say 'here we are, we've got a message for you and you need to start listening'."
But Norma Lane, St John director of operations, said employees wearing mufti posed a "genuine health and safety risk to patients" because it prevented them from being identified by the public as emergency workers.
Disciplinary action would be a last resort and St John would continue transparent talks with the union and its members, Lane said.
"It is helpful that the unions have clarified that their outstanding issues do not involve fatigue but relate specifically to the term of the agreement, pay and leave."
Blacklaws said further action, such as a paperwork ban, may follow that would make it difficult for St John to bill people, but wouldn't compromise patient safety.