Don't know why (if they don't) the NRL don't do a WA / SA / Tas / NT / PNG membership.
Send out a season guide, a NRL pass card that gives you access to 3-6 games wherever NRL is played, some footy cards, info about that region's Jr league etc, and access to bonus content on NRL app filmed and created specifically for these expansion markets without a current NRL team.
People will join if they feel part of something and if they don't have to back a team, the NRL can provide all info to these people.