The Northern Ireland Ambulance Service (NIAS) spent £ 3.2 million on private ambulances in 2018/19, the figures revealed.
The ambulances, led by private companies and voluntary organizations, were sent to 8,514 calls made to 999.
NIAS said private ambulances were needed to meet the demand for services.
The Unison health union said that increasing the use of private ambulances was worrying and was indicative of problems throughout the health service.
An ambulance service spokesman said: "NIAS has contracts with a number of independent ambulance providers to support our daily activities at a time when, as a result of increased demand for our service, our ability to respond to calls is challenged. ".
They are used for routine and pre-planned calls, he said, but can be deployed in "low-level emergencies" if NIAS assesses that it is safe to do so.
Contractors include the Red Cross, the San Juan ambulance and the Order of Malta.
NIAS says the deployment of independent ambulances "allows us to protect our paramedic and EMT response for those calls that are immediate or life-threatening."
The NIAS spokesman said: "Patients who need our service can be sure that NIAS will provide the most appropriate response to meet their clinical needs."
At the end of last year, a review of current NIAS resources identified the service "required by more than 300 additional employees," he added.
"The trust is currently developing a business case that will be submitted to the Department of Health for consideration," the spokesman said.
Unison said the use of private companies was indicative of problems in the health service of Northern Ireland.
"We have been raising serious concerns about the use and increase of private sector ambulances for some years," said Patricia McKeown, Unison regional secretary.
She said the concerns "derive from insufficient investment in the health service as a total and poor workforce planning."
She added: "They are part of the same decision-making process that has led to the current personnel crisis and the general expansion without control of personnel and agency spending throughout the system and has also contributed to the current industrial action."
& # 39; Considerable pressure & # 39;
The agency that decides where much of the health budget is spent said that private ambulances are currently a very necessary resource.
"When NIAS is under considerable pressure, the use of private ambulances provides the much-needed additional capacity," said a spokesman for the Board of Health and Social Assistance (HSCB).
The spokesman said that while work is being done to expand the NIAS ambulance workforce, "some private capacity is likely to be required in the meantime."
The HSCB spokesman said private firms were used to answer calls that are not emergency and "and occasionally in response to emergency calls that have been classified as routine and are not life-threatening, which are the majority of 999 calls. "