Detectives are appealing for two men who came to the assistance of a couple being threatened in Perth's southern suburbs, to come forward.

Police say the male and female victims, both in their early twenties, were confronted by a man with a knife in Como on New Year's Eve.

The offender then forced the victims to drive him around in a grey Toyota Corolla.

Two men intervened while the car was stopped at the intersection of Manning Road and Leach Highway, after hearing cries for help.

The offender was dragged from the back seat and the victims drove away.

Police are urging the two men who were driving a white Toyota Hilux and a white Toyota Landcruiser, to contact them.