A man has been arrested after allegedly stealing a car from a food delivery driver in Port Noarlunga Thursday afternoon.

The delivery driver left his car running while making a delivery on Petersen Crescent about 1.40pm on Thursday 25 March. A man approached the driver and threatened him with a knife before jumping into his Mitsubishi ASX and driving off.

The victim was shaken but thankfully wasn't physically injured.

Patrols quickly attended the scene and with the assistance of Polair, found the vehicle abandoned nearby.

The suspect then slid down a 30 metre cliff at Port Noarlunga where he was met by police and subsequently arrested.

A 25-year-old man from Noarlunga was detained under the Mental Health Act and taken to the Flinders Medical Center.  Investigations are continuing.