Two people are being airlifted to Perth after a horrific dirt bike crash in the State's South-West this morning.

The RAC Rescue 1 helicopter was sent to the crash scene on Old Coast Road near the Armstrong Hills Road turn-off at Lake Clifton about 10am.

It is understood the two bikes collided in Yalgorup National Park and the riders were seriously injured.

The emergency rescue helicopter is expected to arrive at Sir Charles Gardiner Hospital about 11.30am.

Old Coast Road between Armstrong Hills Road and Old Bunbury Road is now re-opened to motorists.

PERTH JAYNE RICKARD