Technically neither Canada or Mexico reset the VWP, so it doesn't matter how long you're there for - you will still need to convince the customs and border protection officer that you're not trying to cheat the system:
If you go to Canada and Mexico or the Caribbean, and while you are
there, your initial 90-day period of entry expires, but you need to
come back in to the U.S. to fly home, you may encounter a problem. The
terms of the VWP are very clear - it is only to be used for
occasional, short visits to the U.S. If the CBP Officer thinks you are
trying to "reset" the clock by making a short trip out of the U.S. and
re-entering for another 90-day period, you can be denied entry. (If
that happens, you will have to obtain a visa for any future travel to
the U.S.) In order to be re-admitted to the U.S. shortly after a
previous admission expired, you will have to convince a CBP Officer
that you are not trying to "game" the system.
However, outside of those two (plus the Caribbean countries), returning to the US from another country will let you reset the VWP:
If you visit other countries such as England or Costa Rica, then
return to the U.S., your re-entry will be considered to be a new
admission (thereby restarting the 90 day clock), rather than a
re-entry from a contiguous country in the course of your initial
visit, and the admission inspection may be more strenuous. The Officer
inspecting you will want evidence that you intend to go back home to
your country of citizenship to live as opposed to returning again and
again to the U.S. after visits to other countries.
They do note at the end there that you may have a burden of proof to show that you don't plan to/are not using the VWP to reside in the US and intend to return back to Australia in your case.
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