You will have to collect your bags after immigration and take them through customs to the domestic transfer desk. Ask there if there are any seats on an earlier flight.

I've found they will usually accommodate your request if seats are available as there are hourly flights SYD-BNE and it frees up your original seat for someone else.

Keep in mind if you've made a special meal request it probably won't transfer to the new flight.

If you have appropriate One World status the Qantas Club in the domestic terminal is quite pleasant at that time of day with a nice view of planes moving around and good food.