While I can understand the Oz decision, our role doesn't exactly make sense. As Theresa May noted in Parliament yesterday, would we really contemplate going to war with China over Taiwan or anywhere else within that neck of the woods?
The pact is about support for Australia. There was carefully no mention of China, let alone Taiwan, Korea or Japan during the television briefing (which might worry Japan and Korea more than Taiwan).
Australia stepped into support the UK in Europe during two wars, I would think being prepared to do the same thing if heads ever come to a head in their region would find public support.