St John is failing to get to emergency callouts in Auckland and some rural areas inside the time limits set for it, according to an internal report aimed at improving its service.
The pressure the ambulance service is under is revealed in a report for a project designed to secure a boost in taxpayer funding.
It comes as Labour health spokeswoman Annette King says she wants to see the emergency part of the ambulance service fully funded.
"The emergency service is a core service. It shouldn't be relying on donations."
St John is funded for 80 per cent of the money needed to provide services to the public - with the exception of accidents - and is forced to seek donations and grants to meet costs.
A bid for increased government funding through the "demand project" has seen St John chief executive Peter Bradley attempt to show how roster changes and new deployment plans could lead to service improvements.
"From the Government's perspective that was us evidencing that we're not just asking for more money for more people. We're doing things ourselves to be more efficient. That in itself is not enough - we need more people," Mr Bradley said.
So far, 20 of 70 roster changes had been introduced and deployment plans had "demonstrably improved performance".
He believed St John had successfully made a case for extra funding, which would lead to more staff.
The reports from the "demand project" used data from September 2012 to May 2013 and show St John's overall national response times meet the Government's required performance levels.
But they also show St John misses response targets for the most serious incidents in Auckland, where overworked ambulance staff get to just 46.5 per cent of life-or-death incidents inside eight minutes and 94.6 per cent inside 20 minutes. They are meant to get to at least half of those incidents inside eight minutes and 95 per cent inside 20 minutes.
They also struggle with less-serious emergencies, making just 68.2 per cent of callouts in under 20 minutes, against an 80 per cent target.
St John does meet its targets for urgent callouts in central Auckland but misses them in the Waitemata area, including West Auckland and North Shore, and in the sprawling Counties Manukau area.
Responses in remote areas are particularly low. In areas as diverse as Benneydale in the Waikato, along the East Cape and in the Far North, ambulance staff struggle to cover long distances to reach people inside the benchmark times.
Ms King said fundraising should not be necessary for the emergency service and she wanted to extend the fully funded part of the service from accidents to medical emergencies. She said it was an anomaly that St John was funded for just 80 per cent of its needs.
"It is a frontline health service and that's why we should treat our ambulance service as part of our core health service," Ms King said. "I would like to see us fund that part of the health service fully."
Federated Farmers president Bruce Wills said those living in rural areas were keenly aware of the need for timely ambulance response.
"We rely on them. We do expect a prompt response from our emergency services."