You have allowed yourself plenty of time.
I would be more inclined to go for a 9.00/10.00am departure.
SYD/MEL airports are now very busy early morning, to give you an idea I usually get through MEL very promptly in 30minutes without baggage, but last weekend it took 90 minutes with 5 lines of passengers waiting in the baggage hall, just to get to the customs check point. This was the hold up, once there they pushed me through a side door, but most were on their way to be checked for foodstuffs. To make life easier for yourself, do not bring in any food or seeds of any description, and if there is anything on the declaration card that you might say yes to, discard it in the bin outside of the plane and state NO. On exiting customs you will notice signs to Qantas domestic, follow these. It is true that Qantas will not put you on an earlier flight, but do not let that discourage you from asking. Most times they say no to me but every now and then they will throw me on an earlier flight home.
I would say, get to BNE, settle in and work out your itinerary from there. OW tickets domestically do not ring any alarm bells, but expect security checks prior to entering all our domestic terminals, usual culprits like water bottles toe nail cutters and large bottles of after shave. Enjoy your flight, Brisbane is warm and sunny at the moment, daytime temp of 27, regards bkkrop