Demand to and from Australia is very high. Most airlines are flying full, especially in J and F. Sone people are even flying SYD-MEL/BNE/PER just so we can get out of the country on our preferred airline and class.
I think the question is more why SYD hasn’t been given an extra rotation or two.
At peak travel periods it’s impossible to book flights. A friend had a wedding in Nov and all of the family based in Dubai had difficulty finding seats in any class and were calling every day. This was before School holidays.
I believe it was similar for most airlines.