The 16-year-old boy was driving when he lost control of the car 140km east of Fitzroy Crossing about midday on Saturday, killing three people, including two passengers who were thrown from the vehicle.
The silver Holden Commodore rolled and the driver, another 15-year-old boy and a 28-year-old man were all killed.
A 16-year-old girl received minor injuries and was taken to Broome Hospital.
Police said the occupants were all believed to be from Fitzroy Crossing in the Kimberley, in the state’s north.
A man and woman, both aged in their 20s, were killed when their Ford Falcon sedan collided head-on with a Toyota Hilux ute west of Paraburdoo, in the Pilbara region, on Sunday morning.
A man and woman, both in their 30s, in the ute went to hospital with minor injuries.
A 37-year-old woman died after being thrown from her car near Bunbury, south of Perth.
The Boyanup woman was driving on Saturday night when her car veered off Boyanup Picton Road and rolled several times.
The seventh fatal crash involved a Toyota Prado that left the road and hit a tree on Bebakine Road, Jennacubbine, northeast of Perth, on Sunday morning.
The male driver, believed to be in his 40s, was killed in the crash.