Police are investigating an alleged sex assault against a female bus driver in East Vancouver, and the victim's union has offered a $2,000 reward for information leading to the arrest and conviction of a suspect.

Canadian Auto Workers Local 111 president Don MacLeod said the victim was driving the 22 Knight Street route at around 7 a.m. on Jan. 1 when she was "sexually groped on her bus" by an unknown man who then fled the scene.

"We are not able to give further information at this time but we hope this $2,000 reward and the bus security camera videotape will lead to a quick arrest," MacLeod said in a statement.

Police are in possession of the security tapes, but have yet to release suspect images.

MacLeod said the incident marks the fourth sex assault on a female driver since 2009.

Of the 3,600 bus drivers employed by Coast Mountain Bus Company, about 12 per cent are female, MacLeod added.