The Fredericton Fire Department is investigating what led to an unprecedented absence of firefighters on Wednesday morning.

Philip Toole, the city's fire chief, said he was notified by a senior officer early on Wednesday that 16 of 21 firefighters who were supposed to be on shift called in sick between 5:30 a.m. and 6:30 a.m.

Toole said the level of staff members calling in sick was "unusually high."

"We started calling individual firefighters who were off to come in. Our number one priority is public safety so we made sure we had the numbers that are required to man all four fire stations on duty at 8 a.m.," Toole said.

The city needs at least 17 firefighters on duty to cover its four stations.

Firefighters who were scheduled to be off were called in, while others were asked to work overtime.

Bringing in these firefighters — paid in overtime — cost taxpayers $6000.

Toole said he notified Fredericton's city administration of the high level of illness at the fire station.

The fire chief said he was not aware of any similar problems with ill firefighters for Wednesday night's shift.

"We have no indications at this time that we will be in the same situation this evening, but if we are we will take the step necessary to make sure we have enough firefighters on duty to safely cover the city," he said.

It's not clear yet why so many firefighters called in sick, however, Tuesday night, city council voted to exclude overtime hours from pensionable earnings for city workers.

City councillor and finance committee chair, Jordan Graham said if the firefighters were calling in sick as a job action, there will be fines levied.

"We're going to be making sure that we put out the appropriate resources so that if this wasn't caused by sickness, that the appropriate price will be paid," said Graham.

Wayne Knorr, a spokesperson for the city, said all the absent firefighters have been contacted and they've been asked to each provide a doctor's note prior to returning to work.

Blair Sullivan, a union representative for the firefighters, said he's sure the firefighters did not call in sick as a co-ordinated job action.

Sullivan was home sick when contacted by CBC News.

Fredericton is struggling to erase its pension fund's $37-million deficit.

City councillors argued Tuesday that taxpayers will save $115,000 annually by eliminating overtime from pension calculations.