Ciaran de Burca told MLAs that more than £500,000 was raised from fines between 22 June and 16 September.
He said extra signs have been erected in a bid to reduce the number of drivers getting caught out.
He was questioned about the scheme by the Regional Development Committee.
Mr de Burca told MLAs that he and other staff at Stormont's Department of Regional Development (DRD) "didn't think we'd raise this level of fines".
He said 18,000 penalty notices have been issued since the scheme was introduced.
About 64% of the fines have been paid within 14 days and were therefore charged at the cheaper rate of £45.
The committee was told that extra signs have been put up on Donegall Square East, which was the camera catching the most cars.
That bus lane, which runs along the side of Belfast City Hall, had been clocking more than 250 drivers a day in the bus lane, but the figure has now fallen to 120 per day.
Castle Street had the second highest number of offences with more than 160 drivers a day being caught when the fines were introduced.  That has now fallen to 45 per day.
The committee was told extra staff had to be brought in to process the higher volume of penalty notices than had been expected.
Money raised will first of all go to the maintenance of audio visual equipment on busses, the bus track system and extra security at park and ride car parks.
Mr de Burca said the scheme was not a money making operation.
However, he admitted that the department's finances are "in the red".
"Any few shillings to go towards that would be helpful," he said.
Mr de Burca is the director of the DRD's transport projects division.
The man in charge of Belfast's new bus lane cameras has said he is surprised by the amount of money they have gathered so far, but denied that the scheme was intended to raise revenue.