I reckon one of the key criteria for clubs continued existence in the NRL should be to have a stadium that holds at least 20k and to be able to fill it routinely. I think if a club can't do this it doesn't deserve to exist in top level competition.
I'd be giving this to the clubs as a target over the next 3-5 years, and giving them free reign to do just about anything to get these sort of crowds, except for giving away freebies. Drop their prices, family discounts, creative membership packages, promotion including club level and active NRL promotional support, etc etc. I'm a big believer in 20k @ $15 average is better than 10k @ $30 average. Other things that would help are good public transport to and from the game, not ripping fans off at every opportunity once inside the stadium (or even the car park).
Some of the stadiums would need upgrading but then if a team wants to play in the top flight they should not flinch at doing this. If they need to raise money to do it, thats what their management and board are paid to do - lobby for govt/council support and funding, arrange funding sources and generally make the club a commercial success as well as an on field one. If the current management and board can't do the necessary, get people who can do it.
... This game will sell out so buy tickets early everyone.
