Upvote:6
You have not yet entered the United States on this trip. As an Australian, you are eligible to apply for entry under the Visa Waiver Program by obtaining an ESTA.
There is a rule regarding entering the US on the VWP and taking short trips to nearby countries, but since you have not yet entered the US that rule does not apply to you. From the above linked page:
If you are admitted to the United States under the VWP, you may take a short trip to Canada, Mexico, or a nearby island and generally be readmitted to the United States under the VWP for the remainder of the original 90 days granted upon your initial arrival in the United States.