MCT (minimum connection time) depends on the individual airports and whether you are connecting Domestic-Domestic/Domestic-Intl/Intl-Domestic/Intl-Intl.

40 mins for MCT in MEL sounds about right. It's not a big airport. I'm pretty sure your bag will make it.

There are published tables indicating the MCT for various airports. Unfortunately I don't want to subscribe to the IATA one for mega $$$$ so I don't have a listing of MCT for each airport.

Easiest way to deal with this is to go to ADL and tell the checkin person that you're travelling to SYD via MEL. They'll then check and tag your bag through.

For additional security, phone up Qantas and link the two itineraries together.