The cheapest is def tiger airways,leave perth 9.30 pm arrive sing 3 am depart to phuket about 8 am.We are doing this tomorrow night and return flights is costing about $620 inc 5kg extra baggage on the way back.
Brunei has flights but more expensive and some flights have a wait in brunie of about 11 hours, and thai can fly you direct to phuket but on the return you fly to bkk and then back to perth,not sure of the lay over in bkk.
Ted