Main Roads has also closed South Western Highway northbound, south of Boyanup.
A man in his 20s is being rushed to Royal Perth Hospital by the RAC rescue helicopter after being hit by a truck.
Around 7:30 this morning the truck struck a pedestrian at the South Western Highway and Shenton Road intersection in Boyanup.
The victim sustained head and back injuries in the crash.
Police remain on site with traffic management in place.
Main Roads advises motorist to seek an alternate route.
Free supplies, health checks, advice for Mandurah pet owners
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Rockingham Detectives ask for public's help to find wanted man
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Pedestrian struck by car in Warnbro
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LISTEN: WA Police release triple-0 call from shooting murder of bikie boss Nick Martin at Kwinana Motorplex
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Police pursuit comes to crashing end in East Rockingham
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Tensions boil over in Rockingham council chambers as motion to reinstate weekly red bin collections is defeated
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WA's first public National War & Service Animal Day commemoration to be held at Dawesville War Memorial
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New road safety laws to be named in memory of Furnissdale hit-and-run victim
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UK nuclear submarine docks at Rockingham's HMAS Stirling
A UK nuclear-powered submarine has arrived in Western Australia, docking at Rockingham's HMAS Stirling at the weekend.