On Saturday night, 37 crews covered the whole of Northern Ireland, compared to the usual 52.
That was three crews short for every one of the five operational areas and 70% of the normal staffing levels.
The service said the St John Ambulance was put on standby to handle non-emergency calls to help ease pressure.
It blamed a late surge in staff calling in sick in the hours running up to the start of the Saturday shift and that it was working to ensure the incident didn't happen again.
A spokesman said he was not aware of the problem harming any patient.
It is the first time we have experienced this level of dropped cover.
Brian McNeill, NIAS
Head of operations for the NIAS, Brian McNeill said: "We can't guarantee this won't happen again, I can guarantee we will make every effort to ensure it doesn't happen again.
"People were off on annual leave, people were off on sick leave and some people were granted casual leave.
"Though we did cancel casual leave, we then experienced a further increase in the number of people who requested sick leave.
"We normally would rely on staff to assist in providing cover to allow for casual leave and short term sick leave.
"Our staff worked extremely hard to provide cover and as far as we are aware, there is nothing to suggest that any patient came to harm.
"We worked proactively to address the issue, we had all priority shifts covered and engaged with the voluntary ambulance services which is a normal practice right across the UK."
While we understand that this was unexpected, and that the NIAS is taking a number of steps to mitigate against a recurrence, the Department finds the situation on Saturday to be unacceptable.
Department of Health
Mr McNeill added: "We are reviewing our processes and we are recruiting both paramedics and emergency medical technicians to cover vacancies to ease pressure.
"The organisation is under pressure in terms of the demands the public is making on it and certainly I would acknowledge the staff do work harder in meeting the demands.
"It is of grave concern and 70% of cover is a pressure for everyone in the organisation.
"But patient safety was not at risk, we provided a safe service, though it was slower than we would have normally provided.
"There were delays and for those that had to wait we do apologise and hope that we were able to meet your needs."
In a statement issued on Monday, a Department of Health spokesman said the reduced service was "unexpected" but also "unacceptable" and they are looking to the Northern Ireland Ambulance Service to work with the Health and Social Care Board to prevent it from happening again.
Unison believes this will happen again and it will cost lives - it is that serious.
Trade union Unison, however, has said it fears a repeat of Saturday's lack of cover.
Eoin Stewart from the union said: "Patient safety had to be at risk, they didn't have enough ambulances to cover.
"It is totally unacceptable, but I don't know what people expect.
"When we have these financial pressures on the health service and we are told by our ministers that you have to cut a certain amount of money, ambulances will be dropped.
"I can't blame staff for not taking on overtime to cover, they are working long hard shifts and they need time off.
"The last thing they want to do is work seven days a week.
"You should have enough staff to cover holidays and sickness.
"We need to put money into the health service, we need to invest in staff put more ambulances on the road and stop the financial constraints there are on the trusts.
"Patient care is not the priority, saving money is."