There are a couple of ways to do it.
Understand though, that as a mere "girlfriend"
the stuff you may have heard about about spouse/ prospective spouse etc
is probably not in play.
She, not you, should speak to an Australian lawyer who is also a Registered Migration Agent.
This lawyer will assist her to make an application, even though she is overseas.
There will be fees to pay, which vary depending on how she makes the application.