The union representing Calgary's transit employees is defending its workers after a report revealed the city's transit authority exceeded its overtime budget.

A City of Calgary auditor's report suggests that worker shortages, in part because employees called in sick, caused Calgary Transit to add nearly 150,000 hours of work in 2011.

Calgary Transit employs roughly 3,000 employees; about 2,000 of those volunteered for overtime in 2011. In the report, auditors say the authority exceeded its overtime budget by 71 percent in 2011 and 29 percent in 2010.

Union spokesman Doug Johannes says there's a reason why Transit drivers call in sick more often than other city employees.

"People getting on and coughing on them and breathing on them and blowing smoke on them and handing them a transfer they've run across their nose or mouth," Johannes said.

"Those interactions are going to cause people to have lost time."

The report found that the average overtime paid in 2011 to operators was $3,596.

Three operators earned more than $30,000 in overtime pay and another 21 operators earned more than $20,000.

Johannes says that overtime was necessary to cope with increased workloads.

"The majority of that work would have gone out during rush hours and it would have been paid at overtime to make sure the work was completed," he said. 

Auditors have suggested that Calgary Transit minimize future overtime costs by optimizing the work force and making sure it meets service demands.