But you were, essentially, posing the threat of tearing up the treaty as part of your argument.
WWII didn't prove anything of the sort. The Japanese bombed Darwin and a min-sub was caught in Sydney harbour. They never had the capacity, or the intent, to invade Australia (the navy wanted to do it, but the army and government opposed the idea and no plans were actually created - they adopted a strategy of trying to isolate Australia to minimise the advantage that it gave to the US). Australia was already heavily engaged in the war in Europe at the time (we had been in WWII for 2 years before the US).
By the end of WWII, Australia had the 4th largest air force in the world behind the US, Russia, and the UK - 25% of which was operating still in Europe and not the Pacific. This did not include Australian personnel operating with the RAF.
The ANZUS treaty has almost zero impact on our trade relations with China - if anything, it probably presents a possibly negative aspect, but even then, I doubt it actually flows into anything tangible. We have extremely good trade relations with China that are mutually advantageous. America as a "backstop" (and it isn't, as I said), offers nothing on trade.
ANZUS was created in 1951 as a response to growth of communism and due to pressure from Australia and New Zealand regarding a peace treaty with Japan (the US wanted to normalise relations with Japan and Australia and New Zealand were not keen).
Could China work its way south? At the moment, no. They literally do not have the capacity to do it. They are working on the power projection, but like the Japanese army and government, they would readily identify the extreme difficulty of doing that today.
I think you are grossly over estimating the ability of China, under estimating the capacity of Australia and New Zealand (and other allies, local and elsewhere), and the dependence on the US - but most of all, you're lacking a serious understanding of the tactical implications of what you're suggesting as a possible threat. Short of nuclear weapons (which wouldn't be a prelude to invasion), the threat is very, very limited for an attack of such a scale.
