A bus carrying about 20 people has crashed on the Kwinana Freeway in Karnup, in Perth's south.

It is understood the bus collided with a four-wheel-drive and crashed through a median strip and into bush near Paganoni Road.

The bus was travelling south when it crashed through a median strip, collided with a four wheel drive travelling north and crashed into bush.

Firefighters, ambulance crews and police are on the scene.

Witnesses who arrived soon after the crash described the scene as chaotic with injured people emerging from the bus.

The bus was heading to Pemberton with about 20 people on board.

Traffic northbound is at a standstill south of Paganoni Road.

UPDATED  12.01AM

Woman dead, others injured after bus crashes at Kwinana Freeway, south of Perth,

"There were two people that we know of inside the four-wheel drive,

unfortunately the driver of the vehicle is deceased and there was a child in the passenger seat of that car

who has been taken to hospital with minor injuries,"

K.J.