It will, of course, depend on whether the Australian government's assessment is that a Chinese invasion of Taiwan represents a problem for Australian national interests. If it does, then the question isn't whether the Australians will help the U.S., it's whether the U.S. will help the Australians.
There is, of course, another explanation. And that is that Taiwan is being set up to go the way of Hong Kong - quietly, and into the night.
Yes, I know, this is heresy. But it does explain all the facts currently in evidence.