Explosion in Vic max-security jail probed

The cause of a small blast in a vacant Victorian prison cell is being investigated.

The "small bang" went off at the maximum-security Barwon Prison near Geelong on Friday, sending it into lockdown an hour earlier than usual.

A Victoria Police spokeswoman said prison officers were doing a routine search of an unoccupied cell when a minor explosion occurred about 1pm (AEST).

The area was evacuated and bomb response unit officers were called.

A Corrections Victoria spokesman said the prison was locked down while emergency authorities were on site.

The small explosion went off while prison officers were conducting a routine intelligence search, and it was being treated as an isolated incident, he said.

"There was no physical damage and no injuries but police, fire and ambulance services have been notified and are attending," he said.

Police and Corrections Victoria are investigating.