They say an elderly man sat next to the girl, who was travelling with a 22-year-old female family member, when the alleged incident occurred.
Police say another woman witnessed what happened and began yelling for the man to stop.
The man was ordered off the bus and the girl and the woman with her also disembarked near the local community centre and called police.
Officers located a man and spoke with him and are continuing their investigation.
Police say they're grateful that the female passenger on the bus interrupted an alleged offence but they would like her to call them so they can get further details.