Australia joining NATO may seem to make sense, Australia has strong connections to nearly all NATO countries, Australia is a strong power, Australia needs allies.
However NATO is quiet a dynamic little group. They are not as a whole going to commit to defending Australia, some partners for sure, some no. What about defending the region around Australia? No, even less, and those that do at a lower level.
Even the US cannot commit to securing to Australias satisfaction the region around her. Sure the US will counter serious external powers (say china or Russia interfering), but internal struggles, turmoil etc are out of its control, the US really doesn't want to get involved in a civil war in asia and her allies wouldn't want that either.
So best thing Australia can do is become a regional centrepeice. An agent who can lead a force of select nations most offering only lower levels of support (coms, intel, supply, a few offering a little more). This deployable, mobile force isnt' enough to overrun an organised and cohesive defence forces but is enough to secure a beachhead in a state of chaos or in a broken, seperating state. Some where forces and land, peace keepers, aid etc.
That and pursue defence alignments with countries on the same page. Australia has strong ties, with UK, US, Japan, Singapore, Malaysia, NZ. Furthering those partnerships would be much more useful than joining NATO.