A convicted sex attacker's escape from prison has left authorities baffled.

A sex attacker nearing the end of his sentence has escaped from a minimum security prison in Victoria’s west.

Barry Dettman, 51, was last seen walking along the Western Highway at Trawalla just after 10am on Sunday after leaving from Langi Kal Kal Prison.

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

Prisoners are sometimes transferred to Langi Kal Kal towards the end of their sentence and it is believed Dettman had less than a month to serve.

Police say Dettman has connections to Bendigo and Werribee and warn the public not to approach him, but to call triple zero.

“When the count was done this morning he wasn’t present,” Crime Command’s Detective Senior Sergeant Michael Phyland told reporters on Sunday.

“We’re certainly determined to locate him so we’d ask that he’s hand himself in,” Det Sen Sgt Phyland said.

Baffled police are unsure how Dettman managed to escape as he had a close to three-hour window in which to pull it off.

The Herald Sun has been told Dettman could not have escaped at night because prisoners were in lockdown until about 8am. The prisoner count was then conducted just before 11am.

Dettman was last seen wearing green track pants and a flannellette shirt is about 178cm tall with a thin build, cropped grey hair and blue eyes.

He has numerous tattoos on his upper arms and forearms including an eagle on his right upper arm.