If I buy a power adapter from the online Australian Apple Store, will it match Australian outlets?

Have a look at: Apple World Travel Adapter Kit

If you get the adaptor kit it works with your existing power supply. You just pop off the end you have, and slide on the new end. It's just changing out the prongs for the correct one needed.

Also works with most other Apple products.

This isn't like a normal international adaptor which sits between the socket and the existing plug - it replaces the existing plug.

Anything you buy from the Australian Apple Store is going to have an Australian power outlet plug. I'd be more wary about buying from Australian 'suppliers' from eBay or similar. You'll probably run into fake chargers there (even with Australian power plugs). There's a good list of differences in the article "Real vs. Fake Apple Power Adapters: 4 Signs of a Fake from Verified.org.

I unknowingly bought one years ago, and it is still working but does get a bit too hot for my liking so I'm always been a bit suss on them.