Have you checked Valuair? Dont know them but their fares seem rather cheap.
http://www.valuair.com.sg/travel_info/perfares_bkk.html
Other alternatives could be e
Thai Airways:
Bangkok - Perth - Bangkok
YHEE1Y (1 year max stay) for THB 45.035 excl airport taxes etc.
Qantas:
Bangkok - Perth - Bangkok
KHEE6M (6 months max stay) for THB 31.495 excl. airport taxes etc.
Now I'm not sure these are direct fligths or not. I just looked up what fares the airlines have for the BKK-PER route in the reservationssystem.
These are the lowest publised fares available from the airlines for staying away 6 months. Could be that you can find them cheaper if you contact a local travel agent that have a consolidator agreement with either Thai or Qantas.