The driver and only passenger died at the scene when their car hit a tree.
Main roads have closed Forrest Highway in both directions from Treasure Road through to Marriott Road.
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Major crash are investigating a crash earlier this morning has seen two people lose their lives.
A member of the public reported the crash about 6:30am.
It's believed a silver Holden Astra sedan was travelling north on Forrest Highway, about 79 kilometres south of Mandurah before it crashed into a tree in the medium strip.
The two victims, who are yet to be identified, were found deceased inside the vehicle when emergency crews arrived at the scene.
Police have revealed they attempted to stop the same vehicle just after 3am this morning on Peel Terrace in Busselton.
Officers searched the Busselton and Geographe areas but failed to locate the vehicle.
Major Crash investigators would like to speak to anyone who may have seen the crash, or the Holden Astra driving in the Parkfield area prior to the crash.
Any motorists who may have dash cam vision of the Holden driving are asked to contact police.
The circumstances involved in the crash will be investigated including the attempt to stop the vehicle on Peel Terrace earlier this morning.
Anyone with information is asked to call Crime Stoppers on 1800 333 000 or make a report online at www.crimestopperswa.com.au
SOUTH WEST - #ForrestHwy closed both directions from Treasure Rd to Marriott Rd, Parkfield - CAUTION - crash
— Main Roads WA (@WA_Roads) January 5, 2018
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