Greyhound are great for the long trips north and east from Perth, but for the southwest Transwa (website
www.transwa.wa.gov.au) are your best bet.
Flights within WA are expensive.
If there are a few of you who can split costs it is probably best to rent a car. Bayswater Car Rental are usually the cheapest but you should watch out for hidden extras that can really add to the cost of the rental.