Westgate to Avalon is only 35-40 mins, Harbour Bridge to Newcastle is 2-2.5hrs. Fancy a 5 hour roundtrip to save 50 bucks? You would be surprised the amount of people who live/holiday closer to NTL vs SYD yet still use SYD. Just like the xx amount of people who drive past Avalon on the way to Tullamarine. A Secondary airport will only work in Australia if it's right in the thick of it, that being a metro population zone. Avalon has been a failure, sadly the Victorian taxpayers are funding 10 years of Jetstar losses (and Andrews propping up old mate Fox- that is a whole thread in itself) to keep it going until later this decade.
Bonza might have some luck using MCY as a more secondary option to BNE for those situation between the two, but they will need frequency to make it work. Daily or Twice a day. 2-3 a week just doesn't cut it, and they will continue to drive to the major cities instead. Getting stranded due cancellations, delays will become a issue, just ask all those Avalon passengers who get cancelled flights and pushed to Tullamarine.