Melbourne Airport transfer times, International to Domestic

You can walk between international and domestic. If your international flight is on time that should be ample time however if you're delayed you could be pushing it.

Recent security incidents could mean that getting through security may take longer so that is something else to take into account.

Is the booking all on the one booking reference? If it is if you miss your flight they will put you on a later one.

I meant "are both flights on the one booking reference".

Assuming it's on one ticket then you'll be fine. Qantas is extremely flexible with travelers arriving from international flights and connecting onto domestic services. If you arrive earlier or later then they'll also help out by switching you to a different flight.