I am travelling from Bangkok to Melbourne via Sydney with a 2 hours 20 mins layover. The journey is on a single ticket. From BKK to SYD it is Emirates code share with Qantas and from SYD to MEL it is Qantas. Do I need to collect my bags in Sydney? Will I have to go through quarantine? Is the layover time enough?
You will go through customs and immigration on your first arrival into Australia (so, Sydney). On the second flight, there will be no way of separating international passengers from domestic passengers and so they cannot really do those checks at a later date.
Generally, you will see immigration first, collect your bags, go through customs etc., then recheck your bags for your next flight. Qantas have a specific set of instructions for transfer in Sydney. You will need to get a shuttle bus between terminals.
In Sydney, there will also be another security check and you won't be able to take liquids, gels, etc, through, so don't buy any duty free until after you're past security!