If you go for tr60 via Sydney apply for pre-approval online it took less than 24 hours for my OX visa then express postage to and from Sydney from Perth all less than a week. Canberra is to slow
Google or ask here when you know where in Thailand you are and can do the extension when you want but don't leave it to the last day. TR60 maybe easier for you
Australians enter on visa exempt entry stamp into our passport and can extend for 30 days at immigration in thailand. Or go online thai consulate Sydney do a TR60 visa pre-approval application, when approved express post passport and papers to Sydney consulate if you have time before you travel, cost is similar to getting the extension in thailand plus TR60 can be extended for another 30 days as well.