As an Australian student who wants to end up in academia, should I do a PhD or postdoc somewhere other than Australia?
My partner did undergrad and grad degrees in Europe, and formed the dream fairly early on to do some sort of study in the US at some point.  That did eventually happen, in the form of a post doc.  My observation has been that as a dream it was a positive life influence.

Dreams of this type are usually most constructive if they are left rather vague.

When thinking about where to apply for grad school, you must consider


funding
fit of school and department
fit of culture (place where school is located)
family considerations having to do with possible relocation


These are all things you can start thinking about now.  There are many strands, schools of thought, etc., within a field of study.  As you progress in your studies you will learn a lot about what intrigues you, and that will help you guide your choice of schools to apply to.

I can say that based on my own experience, I would not expect that your being a returning student would hinder you in the U.S.  I can't speak for other countries.