A 21-year-old man had his arm cut and his car almost stolen after trying to help a man who was lying on the Road in Wembley Downs.

Police spokeswoman Ros Weatherall said the man was driving his Toyota Landcruiser south along Weaponess Road when he spotted a man lying on the street as he approached Dover Crescent.

He pulled his car over and walked up to the man to check on him.

At that moment the man on the road rolled over and brandished a knife.

While still lying on the ground he then slashed the Good Samaritan's arm, threatened him and demanded his car keys.

A scuffle ensued and the knife-wielding bandit ran off.