Yeah, I've lived in Australia and the US. The fine isn't big, the attempts at suppressing voter turnout or inconvenience aren't there, and the privacy of the ballot box means that even if you turn up, and put a ballot in the box, no-one's stopping you writing "You all suck" and not casting any preferences.
But at that point you are choosing to explicitly express your non-support of the candidates. That is still more meaningful than simply not showing up IMO
The Australian system is certainly not perfect, but I think the indirection of the "leader of the party" choice and Prime Minister is helpful - it promotes a greater willingness of the party members to say "Okay, you're hurting -my- re-election chances now", with the result being, whether the ultimate motivation is self-interest, you often (or much more when I was younger) would see no-confidence votes and leadership changes, certainly far more than in the US, where I am now. And signals like you mention, "explicit non-support" seem to carry greater weight.