Specialist officers have been combing the scene at the Jarrahdale State Forest south-east of the Western Australian capital.
The girl was thrown from the quad bike, which she was a passenger on, about 5pm yesterday.
Emergency services rushed to the rural area to help but she died at the scene.
WA Premier Rodger Cook said police were investigating.
“She was a passenger on the quad bike. The quad bike rolled and these awful set of circumstances occurred,” he said.
The 34-year-man behind the wheel was taken to Armadale Hospital for minor injures and an alcohol assessment.
No charges have been laid.
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Police now want bike riders, hikers and members of the public who may have seen the duo before the crash to get in touch with Crime Stoppers to help investigators piece together the tragedy.
So far this year eight Australians have been killed on quad bikes.
Cook warned people to be careful.
“These quad bikes and other tools are great outdoor adventure apparatus but they need to be treated with a great deal of respect and care,” he said.
New figures reveal a spike in the number of deadly crashes on WA roads.
The state’s road fatality rates are well above the national average, soaring by 12 per cent compared to last year, an increase only matched by Victoria — leading the nation — with numbers up by 13 percent.
By Sunday, 81 lives had been lost on WA roads.