I think both the NRL and the relevant state transit authorities have a say in it.
There were RWC number plates just like there are limited edition plates for other things (even the beautiful New England region of NSW). My advice: buy a sticker every year for your window, and one of those little scraper tools to take it off at the start of each season when its cracked and faded.
Moses is right: number plates like that are for boganians.