Hi Sean,
Aussie Trev is correct, you can just show up and pay your $20 to display in the overflow (non club) area.
Alternatively, you can join up with the mustang club (email treasurer@nsw.mustang.org.au) for a $60 annual fee, and you can then display in the main Mustang Owners Club of Australia (NSW) display with around 100 other mustangs.
Once you're a member, the club will provide your ticket without charge. There are also heaps of other member runs/shows during the year, the availability (for period correct cars) of CRS-H rego, a monthly club magazine and many more member benefits, to say nothing of 750+ like-minded people, most of whom you'll end up being friends with!
Cheers,