A woman was robbed at knifepoint while waiting for a bus at a south-west Brisbane suburb overnight.

About 8.15pm on Tuesday, a man threatened the woman with a knife and demanded her phone at a bus stop in Forest Lake, about 20 kilometres south-west of the CBD.

He ran off down Forest Lake Boulevard after she handed over her phone.

Police say the man was a stranger and the woman escaped physical injury.

He was described as having a thin build, wearing a black jumper and track pants.

Police said they found the phone in a rubbish bin in a park on the corner of Joseph Banks Avenue and Woodland Avenue, Forest Lake.

Police are calling for witnesses or drivers with dashcam footage.