Domestically operational upgrades with Qantas are very rare. Likely because because they can fill their business cabins with paying passengers, points upgrades, on departure upgrades with points and staff travel. I have had a lot more sucess on international flights.
Out of about the 200 domestic flights I have done with Qantas I have only received two true opups. Late last year Qantas were giving out some free upgrades due to the grounding/disruption. I received 5 free upgrades which I don't consider opups.
Qantas are very good at yield management. When a flight starts to sell out the price will start to rise. Making people consider taking other cheaper flights. Thereby spreading passengers over many flights, reducing the odds of an opup.
There are a few factors that influence the odds of an opup.
The route you fly - routes with less business traffic are more likely to give opups
Frequency of the route - lower frequency routes increase your odds. On popular routes it is easy for Qantas to justify putting on an extra flight to deal with excess demand so no need for opups.
Type of plane - 7% of seats on a 737 are Business class versus about 12% on a 767 or A330. It is easier for Qantas to fill the business seats on a 737 before having to resort to opups. On the other hand from a numbers basis it is easier to fill economy class on a 737.
Disruptions - passengers from cancellations of other flights and late passengers on connecting flights can be moved on to your flight, which may result in opups for status passengers. But these are hard to predict.
Time of year/special events - school holidays and other events (sports events, concerts) can generate excess demand from passengers who don't normally fly business.
But on the whole best to keep you expectations down.