You won't have lounge access in SYD as you will be travelling domestically on a non-aligned carrier who doesn't recognise your status (Qantas).

Whilst your bags will be checked through to MEL, you will need to collect them in SYD, clear customs and immigration, and then proceed to the Qantas transfer area outside security. Drop your bags, get your BP, get the bus and you're done.

Qantas should give you your AC allowance if you are lucky on your return, but YMMV. You may find that you are restricted to their domestic allowance, since that's as far as they're taking you, at 1x23kg for Y or 2x32 for J if you get a poor agent. I have found that interlining generally means you get what the other carrier lets you have, so it's worth having some documentation there for them to refer to.