Upvote:5
1:40 for an international to domestical connection at MEL is doable but risky, since Immigration and Customs can have long queues at times. However, all terminals are directly next to each other at MEL and you can easily walk between them, so once through the transfer will be quick and easy. There's also a special bag drop area for Qantas connections after Customs, so if you already have your boarding pass you don't need to queue to check in again.
If your flights are on a single ticket, you'll be fine: even if you miss the connection, you'll just be put on the next flight, which Qantas operates between Melbourne and Canberra hourly. If not, though, I would advise you to rebook to add more buffer.