There are several options to get from Perth to Bunbury and return.
Regular trains and buses leave Perth city for Bunbury every day. For timetable details go to: http://www.transwa.wa.gov.au/Bookings/Timetables/tabid/45/Default.aspx
It is 200km from Perth Airport to Bunbury so allow 2.5 hours to make the trip. You can hire a car from major companies at the Airport or, for a much cheaper deal, go to Basywater Carhire which is located 3km from the Aiport. Number is 08 9370 3888 and website is: http://www.bayswatercarrental.com.au/?gclid=CNnM5Lff25sCFRIcawodDm6y_Q.
Note: The venue is 8km out of Bunbury if you have booked accommodation out of town.