*Generally speaking* yes, the QF123 usually does turn around as QF126. Pretty much every day. However I have on occasions seen the QF123 from BNE turn around as QF146 to Sydney, with the aircraft arriving from SYD around the same time as QF145 flying onto BNE as QF126. So be aware that that could possibly happen. But it's not common, and as far as I can see the only days where this might happen are Monday and Friday (as the QF145/146 are only running a few days per week at the moment anyway).
So basically, if QF145 and QF146 aren't both running on the same day, you can pretty much guarantee the same plane will return to BNE.