Whilst I don't have direct experience with Qantas, I have checked a bag during between sectors on British Airways and it didn't even raise an eyebrow, no problems at all.
I was flying from the US to Asia via the Heathrow and my sister wanted to give me a suitcase full of Christmas presents to take out with me, so went landside at Terminal 5, had a coffee took the suitcase, checked in and then proceeded back through to my next flight. I had already got another bag already checked in from the US.
I don't see why it would be a problem.