I live in Victoria and have been wondering this for the longest of times too.
I wish I had of done my research a lot sooner, because I found one somewhat decently sized show that was down here, today actually, and I missed it because I live regionally and only heard of it last night.
VHS (Victorian Herpetological Society) run what I hope is a yearly show - this year it was today 11/3/23 at the Melbourne Show Grounds - if you look them up on socials, you'll find a lot more info - because I missed it, I didn't look too much in to it.
Victorian Herpetological Society Reptile and Amphibian Expo Melbourne Showgrounds
If you found any others, or if anyone else knows of any, can you let me know asap? If you know the frequency of this show too, even better!