A dialect needs a different syntax. US English has some small syntactic differences ("a couple bolts" vs "a couple of bolts") but I'm not aware of any Australian syntax different from British English.
Another US variation is the different meaning attached to the same syntax in "I wish I would have done it" vs "I wish I had done it". No Aussie nor Brit would say the former, except under recent American influence.
Also now unfortunately creeping from US English into Australian is the ridiculously illogical negation of inclusives, as in "they both didn't come" instead of "neither of them came". "They both didn't come" actually means "at least one of them didn't come". The digital logic designers here will know why the US expression is ridiculous.
Yes. Nor does it make it a different dialect.
Some features of US English are older than current British usage, but there are more differences in the other direction.
Clifford Heath.