A man, 21, was forced at knifepoint to withdraw money from a convenience store ATM at Sans Souci on Saturday night.
St George police said the man was sitting on the kerb on Jameson Lane about 10.15pm when another man with a knife approached him.
The man with the knife threatened him and took his mobile phone before demanding he go into the nearby convenience store and withdraw money from the ATM.
The 21-year-old man went into the shop and alerted a staff member.
Both men then walked outside and the armed man ran off across Sandringham Street and up Jameson Lane.
Police later searched the area but did not find the robber.
Detectives are looking for a Caucasian man, aged 16-18 and 170cm tall.
He is of thin build and was last seen wearing a white jacket with a hood, red shorts and sunglasses.
