Philippine Airlines have $550 SYD-BKK
return, with similar fares for other Oz cities. Don't know if these are on special or they just sell some really cheap seats on each flight - they're in the 'budget economy' category when choosing flights. https://www.philippineairlines.com/en
I looked at dates in February and March 2018, which is one of Oz's two airfare low seasons (the other is October).
I didn't book this, but I'd happily fly PAL - hell, I'm an Air Asia / Cebu Pacific / Scoot veteran, any airline where food is served and entertainment provided is in the 'luxury' category.
Tried to convince my daughter to book, but she would rather pay $220 more and fly direct with Thai or Emirates with their current specials. Meh.