Transport Safety Victoria has suspended the accreditation of McNamara Buses following the incident last Thursday.

The young girl had been picked up that morning and was left on the bus until about 2.30pm after reportedly being driven back to a depot.

Children are usually transferred between buses, which then go to different schools and kindergartens.

But the girl remained on the bus, unnoticed by the driver, and was reportedly noticed missing when her parents went to pick up at the end of the day.

The Department of Transport said the operator had been given a show cause notice.

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The company will have to show why their operating agreement should not be terminated.

"We understand the distress this may have caused the child and family, and we continue to work with Transport Safety Victoria to investigate the incident," a spokesman said.

"The safety of all passengers on the public transport network is our number one priority and we will always take action if there are safety breaches."

McNamara Buses directed enquiries to the Bus Association Victoria.

"No comment," a man said when contacted by The Border Mail.

The Department of Education is also investigating.

"The safety of children is our highest priority and we won't hesitate to take action whenever this is compromised," a spokesman said.

"The Department has launched an investigation into the incident and will take necessary action to ensure all children are kept safe."

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