I last received the upgrade email whilst on a MEL-AMM return. I was flying in Y. Received an upgrade email a few days before my return to MEL. The offer was $1400 to upgrade the DXB to MEL leg only... That is the AMM to DXB leg would still be in Y. No lounge access in DXB.
So I think to OP's info regarding $1K upgrades to Business Class being available at Check in is incorrect on departures from Australia.