Ambulance workers and call takers employed by St John Ambulance are taking strike action on Tuesday for the first time ever.
Hato Hone St John has said it will respond to life-threatening emergencies during the 24-hour industrial action, and people should continue to call 111.
Members of the New Zealand Ambulance Association and First Union will miss the first four hours of their shifts between 4am on Tuesday until 4am on Wednesday.
The most critical times during the 24-hour period are set to be 6am to 10am, and 6pm to 10pm, as those are the times when about 75 percent of staff were rostered to start work.
Ambulance Association secretary Mark Quin said ambulance workers were extremely disappointed to be taking this unprecedented action, but they were fed up with the lack of progress after 19 months of negotiations.
"It's gutting. It's the first time we've had to take it to a withdrawal of labour, first time ever.
"This is not a new issue but it's just come to the point now where staff have had enough.
"Never before have I seen staff so united and so angry at St John for letting it get to this."
St John has said it cannot afford to improve on its current pay offer to staff without more government funding.
Director of ambulance operations Dan Ohs said St John was able to put an improved offer to the unions last week, after getting confirmation of increased funding.
However, it still fell short of what unions were demanding, and negotiations broken down.
At that point, St John decided there was no point in further talks at that point, and it was better to focus on preparing for this week's strike action.
The "funders" - Health New Zealand and ACC - were aware of what St John believed it would take to settle the claim, Ohs said.
"They've told us they do not have access to any more money to pass on to us.
"And so our hope is that they're making a recommendation to ministers that will ultimately result in more money for the ambulance service, which we can pass on to our people."
However, Ambulance Association spokesman Mark Quin said unions and their members were "caught in the middle" between the employer, the funders and the government.
"We've got questions around St John's financial management; if you've got the funder saying they've put more money in... our question is - where is the money going? Why hasn't this [been] built in for pay increases to meet at the very minimum inflationary pressures?
"The trouble here is we're being offered a rate that doesn't even cover inflationary pressures."
It appears at this stage the next industrial action will go ahead as planned on Saturday, starting at 4am, with the parties not due to meet again until 29 and 30 August.
Quin said the unions were always open to negotiation.
"We'll meet them any time, we're just waiting for that phone call."