Hato Hone St John Ambulance is pulling some ambulances off the road to rein in costs when staff go on leave or call in sick.
In a bulletin on Friday, management advised staff the service needed to "actively manage recalls" of back-up staff in order to curb expenditure.
St John deputy chief executive Dan Ohs, who heads ambulance operations, confirmed the service was having to tighten its belt for the rest of the financial year.
Other measures included reducing "non-essential" travel and delaying some plans, which would not make "a tangible difference" to services in the short-term, Ohs said.
However, he insisted this would not compromise urgent care.
The Ambulance Association said the "unacceptable" cost-cutting was putting lives at risk.
National secretary Mark Quin said he had already been contacted by worried paramedics in Taranaki and Southland, saying that ambulances had to be parked up over the weekend because they could not be staffed.
"The service is already under significant pressure so this [directive] will only add to it.
"Some patients are waiting five or six hours for an ambulance - that's even with normal crewing and resources on the road.
"If we start dropping resources, that puts on more pressure and raises the risk of people not being able to access an ambulance when they should be."
Ohs said they had kept frontline staff and unions informed on recent developments leading to this situation.
"The number of more expensive overtime shifts being offered has grown recently, and this focus will ensure we are using our available resources effectively and efficiently," Ohs said in a statement to RNZ.
"We do not anticipate the cost savings to have a material impact on the quality of patient care, however, it is possible that some low acuity patients may have to wait longer for an ambulance," he said.
"It still our objective to respond to patients as soon as possible, with immediately life-threatening calls given the highest priority."
They were working to ensure there were enough resources to appropriately respond to any increase in demand, Ohs said.
"Critically, managing our cost base now during a period of (relatively) low demand means we will have the financial depth to deploy additional ambulance shifts during winter."
Meanwhile, Hato Hone St John continued the collective bargaining with the unions representing ambulance staff, he said.
"We are committed to constructive dialogue in good faith and hope to reach a settlement that suits all parties."
But the Ambulance Association's Mark Quin said at bargaining last week, management told the union it had a deficit of $5.5 million for the current financial year.
Despite earlier having agreed to a 6 percent pay rise - subject Te Whatu Ora's approval, Quin said the service now told them it was unable to commit to any pay rise.
In a joint statement to RNZ, Health New Zealand Te Whatu Ora and ACC said there had been "a significant uplift in government funding" for emergency ambulance service under a four-year agreement with providers in 2022.
"Health New Zealand's and ACC's contracts with road ambulance service providers (i.e Hato Hone St John and Wellington Free Ambulance) currently provide around $380 million a year - with an increase of $148m (32 percent) over the past two years.
"Hato Hone St John is responsible for operational matters, including decisions that impact its service costs."
RNZ
