An eight-year-old Spearwood girl is the State's first road fatality for 2013.
Police are also investigating the death of a 17-year-old boy in a possible hit-and-run accident yesterday.
The girl was in the back seat of her 30-year-old mother's Toyota Echo with her two-year-old sister as they drove west on Great Southern Highway from York to Perth.
Emergency services were called just after midnight after reports the car crossed to the wrong side of the road and hit a tree about 6km east of Berry Brow Road.
Acting Insp. Mike Sparkman said initial inquiries suggested the woman swerved to miss a kangaroo and hit the tree.
"It's a tragic way to start the year with a car hitting trees . . . and resulting in an eight-year-old girl dying," he said.
Acting Insp. Sparkman advised that braking in a straight line and hitting the animal was safer than trying to avoid it.
"You've got to drive to the conditions," he said. "If you are on country roads, you have to be aware that there are animals, kangaroos, and you have to drive to those conditions, slow down, be aware of what's going on and if you don't know the area, slow down more."
The mother and her other daughter were recovering in hospital last night.
Major crash police are trying to piece together the death of the 17-year-old found on Ennis Avenue near Safety Bay Road, Waikiki, about 4.30am.
Minutes earlier, police had a call about the youth stumbling on the road.
Acting Insp. Sparkman said his injuries were consistent with being hit by a car but forensic tests suggested he might have been assaulted.
He said police were led to believe the dead youth might have been involved in an incident before the crash.
He asked anyone driving or walking in the area between 4am and 4.30am to contact Crime Stoppers.
"We are just trying to find out what happened in the lead-up to the crash," Acting Insp. Sparkman said. "It may not be linked in any way but we need to know what went on."
Officers found a vehicle possibly involved in the death and a man was being questioned last night.
·A young man was stable in Royal Perth Hospital after falling out of a moving car on Hammond Road, Success, about 9pm yesterday. It is thought the car was travelling at 60-80km/h. His injuries were not life-threatening.