I can't help directly as I have never flown Melbourne to Brisbane on Qantas, but I have flown Sydney to Melbourne many times on QF. Sydney and Melbourne are similar-sized cities, and the frequencies and aircraft types used on both routes are very similar, so my guess is that both routes would have very similar load factors.
Normally, SYD-BNE flights are quite full, around 90% or higher load factor. But it's rare for a flight to be 100% occupied. So with a little luck, your standby ticket should be OK. Flights are hourly, so you wouldn't have to wait too long for another chance if your initial flight is full.