The 3 hour day is a rare thing, but I dont think that its more available in Australia than anywhere else... I know more people with that setup in the states, as an anecdote.
And fully remote in the states has the additional benefit of actual high speed internet.
Well all I can say is that I've worked at 1 startup, 1 agency and 2 very large tech corporates here in Australia, and I've been able to maintain only working 2 - 3 hours a day at each of those companies, as well as maintain a few days not doing anything at all.
And I wouldn't even consider myself top-tier at what I do.
Although to your point, I think Australian work culture is very flexible. You can work a lot if you want. You can also do the bare minimum. I've just noticed that there's no enforced standard, aside from doing literally nothing. In a lot of cases, the few people who do work a lot do end up making up for all the people who don't want to work, so it evens out in the end.
And fully remote in the states has the additional benefit of actual high speed internet.