If you’re not carrying too much with you, I highly recommend taking the V/Line train + bus combo service to the Avalon Airshow. That’s what I did this year and it worked brilliantly.
Last time I went there, we drove. That resulted in over three hours of total driving time — most of that in slow, heavy traffic on narrow lanes behind the airport that went to/from the airshow parking area. And then we had to park a good fifteen minute walk away from the airshow entrance. Not fun. Wouldn’t recommend.
This time someone else did all the driving and I got to nap in a cool, quiet carriage all the way back to the city :)
All this didn’t cost very much either: $13.60 for a return ticket from Footscray railway station to Lara railway station and then a special shuttle from Lara to a convenient drop-off/pick-up point near the airshow entrance. (I bought the ticket in advance, though not online, which is why it cost me more than the online full-fare price but less than the $15 day-of price.)