September is always a good time to buy LEGO. You are over the mid-year toy sale madness and you need something to keep you going until Christmas. It also happens to be when there are always sales on.
Big W’s September school holiday offer is “At least 20% off”. There’s a catalogue full of sets to check out. It’s pain to calculate the discount on everything because it applies to Big W’s regular price. So 20% off one set might be better than 30% off a different one depending on how Big W has priced them.
I can’t actually find an end date for this sale, but I assume it’ll end before the school holidays.