Various comments and reasons upthread about the time needed to ramp up flights. In the most recent case of SYD-BNE QF loaded more flights fairly quickly from a week out (from 7 Dec) from the announced start date (1 Dec) at very reasonable fares ($129+). The first week (i.e. this week) stayed relatvely expensive ($199+). It was rather later that they loaded more flights for this week, and the number of flights BNE-SYD was increased from 4 to 12 on 10 Dec, but only from 4 to 9 on 3 Dec. There are still $199 seats available in two days time.
So my suggestions would be start hunting for flights that are at least a week out from the start date of border reopening, be patient and see what more comes up, and also do that if you want or need to fly immediately after the border closure.
QF have now loaded about 5-6 MEL / PER flights a day. Haven't looked at SYD.