Do I need a USA visa if I come from Canada on an Australian passport?
No, not to enter Canada, anyway. If you plan to visit the U.S. while you are here, the Electronic Travel Authority is OK if you fly in, but you will need a full visa to drive across. Fortunately, it is fairly simple to get at the border. We had Australian friends visiting and went we got to the border, the Border agents pulled aside and our friends went in a completed some paperwork. I think it took ten or fifteen minutes.