You will have to clear immigration in order to collect your bags which, as you've been told above, won't be checked through on Qantas and v/v. Whilst the timings are ok if everything is on time, and Changi is a very efficient airport, you should understand that a significant delay or cancellation of your TK service that leads to you not checking in on time for the QF flight will result in you being declared a no-show and having that entire ticket cancelled. If you wish to travel to BNE you will likely have to buy a new ticket, unless OF takes pity on you and accommodates you on a later flight.