Petrol in a diesel Subaru

You will need to drop the tank, or try using a bilge pump to pump the fuel out, don't use anything electrically driven you may start a fire...

If you started the engine you need to have the fuel lines blow out.

Pull the back seats and fuel senders out, use a drill powered pneumatic pump and empty out into a container. Make sure the drill is far outside the car as electric motor can ignite fumes. Use rags to get the last bits out of the bottom. The bottom of the tank isn't flat to you need to drain both sides.