I think those dates are just before the Spring Break here in Australia in Sept. Christmas is always busy at QANTAS because it is the Summer break for Schools from Mid December to End Jan each year so it would be near impossible to get your points seats I would think
While school vacation periods have an impact, award availability is part of the black art of yield management, of which Qantas is one of the best exponents in the industry.
Qantas tends to allocate a number of seats on most flights/routes when they release award availability at 253 days before departure. Once those seats have been snapped up, they tend not to release any more seats until much closer to the departure date when they have a better indication of the loading.
This means that if you get in early and book before the limited number of award seats have been consumed, you have a better choice of routing. In you case, the more direct and better timed flights probably did have award seats available 11 months before departure. But they were likely snapped up quickly (being direct flights on popular routes). They may review the loadings and if light they may release more award seats in the last month before departure. So if you are looking between 11 months and 1 month out, its quite likely there are no award seats and light commercial ticket sales.
If Qantas kept releasing more award seats just because the original ones wee already "sold" and commercial ticket sales were light, they could possibly fill half the aircraft with award redemptions. If they had released 5 award seats 10 months out, they may have been gone in a few days. Then another 5 a month later and they could be gone the following day. So you could still be looking every week and still see no availability, even though they have been releasing seats.
I have managed to obtain 3 x business class awards in the middle of January for travel BNE-MEL-AKL. That is the time of year when paid business class travel is light but economy cabins tend to be full. I routed via MEL because the MEL-AKL was a 747-400 with lots of business class seats while the BNE-AKL direct flights had far fewer seats in the cabin and hence reduced possibility of finding 3 seats on the same flights. That was booked about 4 months before departure. But there was only business class available and not economy at that time of the year. So it certainly can be done during school holidays.
Also remember that QF award seats are not just available to Qantas FF members. They are available to all OneWorld members as well as some of the partner airlines such as Continental. So the pool of people seeking the award seats is very large, especially for the popular international segments. So you need to get in as soon as they are released of hope more are release in the last few weeks before travel.
Even if Qantas doubled or tripled the number of award seats they made available, there would be more demand than supply on many popular routes.
With plenty of planning, I have not had much difficulty in redeeming my FF points/miles for award seats to the destinations I wanted at the times I wanted (with a little flexibility in routing and specific dates). But it does take some planning and effort to identify options to get what I want.