St John is investigating 19 incidents in the past few weeks when an ambulance was delayed, and looking at the impact that had on the patient.
It comes as demand for ambulances reaches record levels, with unprecedented calls to 111 before the Omicron outbreak has even taken hold.
St John deputy chief executive of ambulance operations Dan Ohs said the charity had plans in place to deal with Omicron demand - including getting an additional 14 ambulances on the road; eight in Auckland and six around the rest of the country.
That would help cope with a surge in demand and be a buffer for any sickness amongst staff.
But finding enough staff is proving challenging - right now there are 150 frontline ambulance vacancies across the country, an 11 percent vacancy rate for the organisation.
Ohs said that was higher than the usual 7 percent vacancy rate, but St John was getting ambulances out the door to targeted levels 97 percent of the time.
A big recruitment campaign is under way and Ohs said the government was putting millions of dollars into helping St John cover those costs, including an intensive training course, which would see 64 new staff graduate in March.
Over the past week, hundreds of volunteers and staff from elsewhere in the organisation have been called on to help and discussions are underway with other commercial providers to step in too.
But Ohs said staff were already dealing with Covid-19 challenges.
"Going to Covid patients, wearing PPE for long periods, the back of the ambulance when you're in PPE is very hot, and so we're finding that our staff are getting very tired and they're needing their days off.
"And so that's contributing obviously to it being more challenging to get staff to come back and do recall."
Amid the staffing struggles are delays in getting to patients. St John is investigating whether slow ambulance responses contributed negatively to patient outcomes in the 19 incidents so far this year.
Two of those involved Auckland callouts where patients died before St John arrived.
Ohs said he was unaware of the volume of concern from staff in the past.
First Union ambulance coordinator Faye McCann was not convinced St John was ready to deal with the Omicron outbreak.
"There are big concerns about the state of the service, staff are already reporting that they're burnt out and this is before Omicron has even hit."
With paramedics recently becoming registered, she said many were finding work elsewhere in the health sector.
"The starting rate for an ambulance officer is below the living wage, you can be working 14 hours with only a 30-minute break, there's a lack of recognition for unsociable hours with the rate being significantly lower than others in the health care industry."
McCann said the range of issues all indicated the ambulance service needed to be taken over by the government.
"The ambulance service is run by charities, there's no other essential service which would be run by a charity, and I think that causes a lot of issues in itself," she said.
In a statement, Health Minister Andrew Little said officials had been working closely with St John, Wellington Free Ambulance and air ambulance providers to ensure that surge planning for Omicron was well supported.
He said he was happy to consider whether appropriate additional support could be provided.
