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Now to your question: You may upgrade only with points and indirectly at that. You can't pay $.
First you must find Business Award availaibity on your flight or a similar one (very difficult on that route). You then either 1) Book it on points and cancel the original, or 2) Contact Qantas Frequent Flyer and get them to rebook in the available business award seat.
NB, this will probably cost you at least an additional 5,000 ff points. Also, for 1), you will need enough additional points for a one Way Business Award (96K).
No way to "upgrade" as such (money or points). As Serfty says, you need to cancel the current award booking and cop the 5000 point penalty, and then rebook as a business class award pending availability.
You may way to try a tool like FrequentFlyerFriend to help locate business class award seats on the route you want.