I did some looking around here, but it's difficult to find a specific solid answer because this is going to vary between the universities, to an extent. But what I've found points to this:
Even though you are a British citizen, you have not been 'ordinarily resident' in the UK, which means you would be considered an international student. It would appear that international status is determined by citizenship AND residence, a bit like US universities. However, universities may be hesitant to respond to your queries about this either way until you were offered a place to study.
Obviously you would not need a visa, but I'd imagine what you're worried about is whether you would have to pay international fees, which are very high.
I'm going to openly admit that one of my sources is Wikipedia, so take it with a grain of salt. You could potentially qualify as a 'home' student in spite of living in another country if you were from:
British Antarctic Territory
British Indian Ocean Territory
British Virgin Islands
French Southern and Antarctic Lands
Saint Pierre and Miquelon
Turks and Caicos Islands
Wallis and Futuna
However this is not the case for you, as you are in Australia.
Here is the list of requirements for British citizens to be considered a 'home student':
Be free from any immigration restrictions (e.g. British citizenship,
exercising EU Freedom of Movement Rights, indefinite leave to
enter/remain, right of abode, free from immigration control (as a
diplomat or member of air crew))
Be ordinarily resident in the United Kingdom
Have been ordinarily resident in the United Kingdom, Channel Islands
and/or Isle of Man for the whole period of the three years directly
preceding the first day of the academic course
The main purpose for the three years' residence in the UK and Islands
must not have been to receive full-time education during any part of
it, unless the student is a European Union citizen (but not a British
citizen) and immediately prior to the three-year period was ordinarily
resident in the European Economic Area, Switzerland or qualifying
overseas territories (see below).
You have not been ordinarily resident in the UK, ergo you would be an international student.
If you want to confirm this, you should get into contact with the international offices at your universities of interest. Only they can answer definitively.
Wikipedia, use caution
UKCISA Fee Status Assessment
Complete University Guide