Regardless of whether the bags are tagged to CBR or not, you will need to collect your luggage on arrival into Sydney and take it through Customs and Quarantine

Once you have cleared Customs, then follow the signs to the Qantas Domestic Transfer Desk ( if you are facing the windows of the airport, turn right and head towards McDonalds ) and you will be able to drop your bags off and , if you do not have one yet, collect a boarding pass ( since you are on a Dash-8, I'd recommend asking them for 1B )
. There is then a bus that will take you and drop you off airside in T3

The entrance to the Qantas Club is near Gate 5; if you show your international boarding pass, then they will v likely let you in

The flight to CBR will depart from gate 15

They may have trouble issuing you with a boarding pass for the SYD-CBR sector since it is so far before departure; if they cannot, then you can try using online check in at the lounge during your stop in BKK

Dave