Possible links between a 17-year-old girl and a man who was fatally assaulted in Mandurah, south of Perth, are being investigated by police.

A 28-year-old man was found in a car park on Pinjarra Road with severe head injuries on Sunday night and later died.

The teenager was found nearby with serious but not life-threatening injuries.

Major Crime Squad detectives, who have obtained security footage from the area, are now trying to establish whether there was any link between the pair.

Media reports suggest the man was set upon while trying to come to the aid of the girl, who he did not know, and was being attacked.

The teenager is in Royal Perth Hospital in a serious but stable condition.