A convicted armed thief has robbed a bank for a second time after escaping from a Perth hospital, where he was about to be treated for a medical condition.

Ronald Rowles, 51, escaped from custody shortly before 1:00pm, according to Perth police.

After slipping past the security assigned to him at the hospital, Rowles later robbed a Westpac Bank branch in Northbridge. The amount of money he has stolen has not been reported, according to ABC News.

Police further told the media Rowles robbed the same Westpac where he committed an armed robbery in June 2010, during which he gave a bank teller a withdrawal slip with demands written on it.

The crime scene has been cordoned off while Rowles is hunted by authorities.

According to ABC, Rowles is described as 170 centimetres tall, with a slim build and fair complexion. He has grey hair and blue eyes, and was last seen wearing a green prison uniform.

Authorities have released a photo of him to warn the public of the danger he may bring.