I recently traveled from Australia to Mexico with a 1 night layover in Miami USA. When I landed in Miami for one night for the connecting flight, I was granted the 90 day VWP Visa. I will be travelling Mexico for 3 months and therefore my 90 VISA will be finished. I wish to travel USA after my time in Mexico.
My question is: When I re-enter the USA, will they give me another VWP visa or will they expect me to return to Australia before being granted another?
You should be fine. The rule is intended to prevent people from using short trips to circumvent the 90-day limit of the VWP, and that is clearly not what you're doing here.
See the State Department's web site (https://travel.state.gov/content/visas/en/visit/visa-waiver-program.html):
Trips to Canada, Mexico, or nearby Islands
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. Therefore, the length of time of your total stay, including the short trip, must be 90 days or less. See the CBP website. Citizens of VWP countries* who reside in Mexico, Canada, or a nearby island are generally exempted from the requirement to show onward travel to another country* when entering the United States.
