A long-serving paramedic has been fired by St John, after sharing information found on the company's intranet.
The union says the documents, which were accessible by any employee, showed St John was not bargaining in good faith.
However, St John says the information was private and should not have been accessed or shared.
On top of one man's dismissal, up to five other First Union members are facing their own fallout.
It comes as pay dispute negotiations between First Union, which represents almost half of the organisation's workforce, and St John reach boiling point, ahead of unprecedented planned strike action next week.
"If you disagree with St John in any way, shape or form and you're a First Union member, watch out," an anonymous paramedic, with decades of experience, told 1 NEWS.
She said workers love their job, but "hate" their employer.
First Union co-ordinator Sarah Stone said: "This is utterly shocking for someone in public health in New Zealand to treat its workers this way"
1 NEWS has seen parts of the document at the centre of this issue and it details a plan to offer a "sweetheart" pay deal to ambulance officers in two other unions, isolating and discriminating against First Union.
St John chief executive Peter Bradley says those who shared the document breached the organisation's privacy.
"It was an internal document for our board to talk about, how we would work with our unions, and how we would fund our pay settlement," he says.
But it was accessible to all members of the organisation. 1 NEWS has been told it was on the intranet for more than 50 days.
"Over 3000 people could have accessed that... It wasn't marked confidential and sensitive," said the paramedic.
Despite what was found, in writing, the ambulance service says there's no plan to discriminate against First Union members.
"We have no desire or intent not to work with all of our unions, or to oust First Union. That's never been part of our plan," Bradley says.
The dispute over better pay for paramedics working unfavourable shifts is still not resolved, with some saying the organisation isn't bargaining in good faith.
"It's very difficult to prove bad faith unless you have a smoking gun, of something that is deliberately deceptive," law expert Bill Hodge says.
"It's pretty close but there is nothing in the statute that says you can't offer a better deal to one union."
Two 24-hour stop work strikes are set to go ahead next week.
"This is the first time in New Zealand history that ambulance officers have ever voted to withdraw their labour," said the paramedic.
Stone said the workers "can't take it anymore".
"They will not put up with this behaviour from St John Ambulance any longer," she said.
"I have to make it really clear: These workers do not want to strike. They just want to go to work and they want to be paid for the work they do at nights and weekends."
Bradley said St John still wants to resolve the dispute.
"We want to find a way through all this... and, of course, we can always do things better."
But Stone said the message the union is getting is clear: "St John doesn't want to find a resolution."