Detectives are investigating following the armed robbery and assault of a man at Biggera Waters overnight.

Preliminary investigations indicate around 8.20pm last night two employees of a fast food store at a shopping centre at Biggera Waters observed a man on the roof of the premises acting suspiciously and called out to him to come down.

The man came down from the roof and he and two other males confronted the employee demanding his personal property.

When the man refused one of the males produced a knife. The man was then punched to the ground and assaulted by the group who stole a mobile phone from him before fleeing.

The 32-year-old victim was transported to the Gold Coast University Hospital for treatment to facial injuries.