The Supernatural Amphitheatre is about 110 minutes drive from Melbourne International and Domestic Airport (Tullamarine) in average traffic.
Avalon Airport is much closer, located just East of Geelong. Avalon has very few international flights, but the domestic airline Jetstar has several flights from many Australian destinations around festival time.
On the first day of the festival you can expect heavier traffic with some delays – here is a guide to travel times to the festival site on the opening day:
Melbourne CBD to festival – 3 hours
Melbourne airport to festival – 3 hours
Avalon airport to festival – 2 hours