Well it looks like you don't have much time to connect another flight to Canberra. I have done it before. I arrived BNE at about 7:00am. There are lots of people linining up at the Custom and Quantitative inspection. They all arrive from Asia, Europe, Pacific, Middle East and USA at early in the morning. It will take a loooong time to get out.

Not sure you have got a Qantas ticket. If yes, you can call Qantas to change the flight time or you can ask ground staff too. It's free of charge because you have Class J Award or use miles to get "free" flight. (must be available before departure). I have done it before.