A teenage girl escaped unharmed after an 18-year-old woman robbed her at knifepoint before detaining her in a western Sydney police station.

The 16-year-old girl was waiting outside Bass Hill police station on Tuesday afternoon, when the woman allegedly threatened her with a knife, demanded her phone before taking her inside the station.

A senior constable was able to negotiate the teen's release and the woman was charged with several offences.

© AAP 2018