Police are focusing their attention on Werribee and Bendigo, where escapee Barry Dettman, 51, is believed to frequent.

Police have confirmed he was spotted in Werribee at 5pm on Sunday. They are continuing their search.

The public is urged not to approach Dettman, but to contact police. He was jailed for charges including indecent acts in the presence of a child, and wilful and obscene exposure.

Dettman was scheduled to be released from jail within a month.

EARLIER: A sex attacker jailed for indecent acts in front of a child is on the run after escaping a regional Victorian prison overnight.

Barry Dettman, 51, was less than a month from being released when he left Langi Kal Kal Prison in Trawalla, north-west of Ballarat, police say.

Victoria Police Crime Command Detective Senior Sergeant Michael Phyland said the man was serving time in a minimum-security prison for charges including wilful obscene exposure. 

It was unclear how and at what time Dettman escaped, but Detective Senior Sergeant Phyland said he was sighted near the prison on Sunday morning. 

"When the [prisoner] count was done this morning he wasn't present and the search has begun from there," Senior Sergeant Phyland said.

"We are appealing for members of the public who may know about Mr Dettman's whereabouts to contact triple zero and inform police."

Senior Sergeant Phyland said police did not believe that the man was armed, but warned people not to approach him. 

He was last seen walking east on the Western Highway, towards Melbourne, about 10am.

Police are focusing their search in the Bendigo, Werribee and Melbourne areas. 

"We are looking at number of areas," Senior Sergeant Phyland said.

"He has a long standing association with the Bendigo area and so obviously we are focused in the area," he said.

"We will be working around the clock ... and we are certainly determined to locate him so we ask that he hand himself in."

Dettman is serving time for a range of offences including indecent acts in the presence of a child and wilful and obscene exposure. 

Dettman is 178 centimetres tall, of thin build and has cropped grey hair and blue eyes.

He has a number of tattoos on his upper arms and forearms including an eagle on his right upper arm.

He was last seen wearing green tracksuit pants and a flannelette shirt.

According to the Corrections Victoria, Langi Kal Kal Prison is a minimum-security facility for "prisoners who require a level of protection".

- The Age