Mel Still answers:

You can usually get good-priced flights to Bangkok with Jetstar up to about 3-4 weeks beforehand if it’s not a peak time (like school hols), but I’m referring to Jetstar’s normal fares rather than special sale fares.

For sale fares, just subscribe to Jetstar and keep an eye out for the sales… There should be a good sale between now and then, but you always need to be quick.

You can keep track of price movements using Adioso:

http://www.adioso.com/search/brisbane-to-bangkok.html

It only covers about 3 months into the future so it won’t help you with September flights just yet, but will give you a good idea of how the price fluctuates.

Also, note that Air Asia has some great deals from Gold Coast to Kuala Lumpur:

http://www.adioso.com/search/gold-coast-to-kuala-lumpur.html

From KL, you can get to anywhere in Thailand really cheaply:

http://www.adioso.com/search/kuala-lumpur-to-bangkok.html

http://www.adioso.com/search/kuala-lumpur-to-thailand.html

UPDATE:
Chaos’s answer is not correct; prices fluctuate a lot, and attractive sales can happen at any time. You’re best to keep an eye on the prices to understand how they move.
It is a good idea to be careful not to hold out too long for a cheaper price and risk the price going up, but if you buy about 3-4 weeks out you should be fine.