As said probably your best bet of a cheap bargain fare is with a discount airline but as also said this can become a milk run if you`re not careful.
One cheap way for you might be to look for a cheap fare to Singapore with Qantas or Singapore Air then fly Tiger Air to BKK.If you want to fly direct to Samui from BKK it has to be with Bangkok Airways.
A price to start you off would be $1100 return Syd to Sing then less than $40 return Sing to BKK with Tiger.Bangkok Air to Samui is around $260 return.Bangkok Air rarely offer specials unless it is on their first or last flight of the day.
There are ways of getting cheaper fares through a travel agent.This usually involves staying at a hotel/resort that say Qantas or Thai Airways deal with through their travel arms, with Thai it`s Royal Orchid, Qantas is I think Qantas Holidays,this way you get what they call a wholesale fare.
Other than that enter "lowest airfares Sydney to Bangkok"in to Google & see what comes up.
You have 9 months & plenty of research ahead
Good Luck
Jamie

