Four separate traffic crashes in Perth within seven hours have left five people dead and another two fighting for their lives in hospital.

The worst crash was a triple fatality near Yanchep, where a sedan carrying four people is believed to have attempted to make a U-turn on Indian Ocean Drive when it was hit by a four-wheel-drive about 7:00pm on Sunday.

Three people in the sedan died at the scene, about four kilometres north of Old Yanchep Road.

The driver of the sedan, a 28-year-old man and two passengers, a 20-year-old woman and 31-year-old woman died at the scene.

The remaining passenger, a 25-year-old woman, is fighting for her life in Royal Perth Hospital.

Five people, including three children, in the four-wheel-drive were taken to Joondalup Health Campus for assessment, but were not been seriously injured.

Meanwhile, a man believed to be in his 20s, remains in a critical condition in Royal Perth Hospital after his car hit a tree at Karnup about 8:30pm, in Perth's south.

Freeway tragedy after sharp lane change

Just hours earlier a 25-year-old woman from Nollamara died when two cars collided on the Mitchell Freeway in Leederville around 4:20pm.

The woman was a passenger in a car in the far-right lane of the freeway when it manoeuvred sharply left, and hit another vehicle.

She died at the scene, while a 24-year-old woman in the other car was taken to Sir Charles Gairdner Hospital.

A 22-year-old man from Mount Nasura died when his motorbike crashed in Pickering Brook in Perth's south-eastern suburbs shortly after 1:45pm.

Police said the motorbike was spotted speeding on Pickering Brook Road, but when officers tried to pull it over it sped away.

According to police, a short time later the rider tried to overtake a vehicle turning left, as another vehicle was coming in the other direction.

The rider managed to miss both cars but hit a power pole.