Their number of ACT members in somewhere between 10-15k. They haven't released the exact number in years, but last time they did release their member numbers from outside of NSW, which it's safe to assume the majority of which would be from the ACT, it was something like 13k (I forget the exact number), and it's undoubtably grown since then.
However their "fan-base" in Canberra is disproportionally supported by expats, and the vast majority of them couldn't be described as GWS fans by any reasonable standard. The minority whom could be described as GWS fans are mainly from younger demographics whom don't know any different, however as soon as GWS leaves (which is inevitable) that support base will almost completely dry up just like it did after the other AFL clubs left over the years.
Furthermore the ACT government is only renewing the contract because Andrew Barr, and a core of ACT Labour, are AFL sycophants. The ACT taxpayer lost fortunes on the last two contracts, and overpaying so much for a handful of AFL games that nobody else would even bid for (not even Tasmania) each year has made the ACT uncompetitive when competing for other more desirable events.
In other words to achieve what GWS has achieved in the ACT you need a highly transient population base, with a government that's willing to literally sell their souls to get the contract (and "free" GF tickets each year...), and even then the support will be at best fleeting. That's without getting into the other major negative effects (which are watered down in the ACT because of the high transience, but are still there just like in Tasmania) like mass disenchantment and frustration with the sport/league after being lead on for years.
No such market exists for the NRL, certainly not Adelaide or Wellington, and even if it did it's not worth the long term negative side-effects.
Besides, the NRL would be absolutely made not to put a club into Adelaide, and it's only a matter of time before the AFL puts a side in Canberra. Aside from Tasmania Canberra is the only new market that is a realistic option for them in the short to mid term, and more lucrative than a lot of the other markets they could/are trying to develop for the long term.