An accident on the Kwinana Freeway in which a car was crushed by a truck caused severe delays to Perth peak-hour commuters on Wednesday evening.

A 27-year-old woman is in a serious but stable condition in Royal Perth Hospital after the crash in Success, which involved three cars and a truck.

The Department of Fire and Emergency Services (DFES) said the woman had been trapped in a car under the truck.

The crash has closed the Kwinana Freeway at Success, south of Perth. ( Supplied: Main Roads WA )

Two men and a child were also taken to hospital following the crash. None of those involved in the crash have life-threatening injuries.

A DFES spokeswoman said it appeared another car involved in the crash had spun off into nearby bush.

The freeway was earlier closed northbound from Rowley Road to Beeliar Drive in Success, causing significant delays for peak-hour traffic, but has since been reopened.

Main Roads had warned of flooding on the Kwinana Freeway at Armadale Road in Jandakot, not far from the crash site.

Perth has experienced heavy rain for large periods of the day.