St John rejects a claim it used bullying tactics to stop workers protesting their working conditions.
Their union originally threatened a ban on paperwork, but altered its plans. First Union is locked in a collective bargaining dispute with St John.
Union members yesterday received a letter from the law firm representing St John. It warned disciplinary action could be taken against anyone not complying with its uniform policy, First Union spokesperson Lynette Blacklaws said.
"We weren't expecting bullying tactics - staff aren't taking much of an action but something they feel keeps everyone safe," she said.
"Ambulance officers are sending a message that they're overworked, nothing more."
St John disagreed.
"If employees make the decision to wear mufti and cannot be identified as St John ambulance officers, there is a genuine health and safety risk to patients, especially children and other vulnerable patients," a statement said.
It insisted officers protesting were under no threat of disciplinary action now, but left the door open for further down the track.
"As a last resort, St John may be required to take disciplinary action to protect patient and staff safety. "We have been transparent in communicating this to First Union and their members."