Hello Jayne, it depends: (a) if you are in a good mood - great thanks, never better - not bad, things are fine/good - brilliant thanks - yeah fine, can't complain (b) if you're not in a good mood - so so, not great to be honest - ok thanks, could be better - not feeling myself lately - awful, I need a break/holiday

You can also say "How are you going?" instead of "How are you?" to an Australian.

It's a very open question, a 'conversation opener' in fact and allows you to say whatever you want. You can easily create a topic to further the conversation.