You know, I can't help but feel this tyre restriction idea is a bit over the top. Why bother? After all, it's an Aussie championship, and traditionally people build the best bike within the rules. The blokes who don't have the money or talent aren't going to be the champion - they'll be there to ride and be part of it. Trying to enforce a level playing field is a mug's game. What next - fuel restrictions, engine restrictions, skill restrictions?
Look, it's dirt track on an oiled circuit. I'd much rather watch 80's model bikes running big wheels/tyres like they did back in the day, or US style flat-trackers with Carlisles etc than to see everything forced onto 18" trials tyres.
Why not just stick with the rules as they are?
By the way, does anyone enforce rule 20.2.1.2 in modern dirt track? Note that this rule does not apply to Classic Dirt Track.