Could the issue then be that Welly is a small city and therefore has the hardcore fan base relative to its size. My home town in Europe is about the same size as Wellington (approx 400,000) and while football is obviously the number one sport, there are also others competing with it for popularity. My hometown team ground holds only 18,000. Over the last ten years the average crowds were (over each season) from 4K (in bad seasons) to 6K (in good seasons). Only twice over that period was the average crowd 10K.
One clear difference I noted while looking over these stats for was the relatively high number of visiting fans of the opposing teams (this is known because they get separate seating by sectors). The opposition fans come by train, bus or they fly, which is not particularly expensive in Europe. Accommodation costs are similar to those here in NZ. But getting to away games in in A-League is not cheap as a regular thing.
Could it simply be that the way to grow the crowds is to look for an A-League wide deal with an airline to have cheapish tickets for the club fans (as a way of sponsoring the league). These should be fenced-off to ensure people actually have to go to a game to get the cheaper airfare. This would be useful for us flying across the ditch to support the Nix although lots of Kiwis live there anyway, but even more important to the West Island team supporters coming over here, with the NZ dollar moving up close to the AUD lately.
I would not mind seeing more opposing team fans at our game, as long as the security can be educated that this is all good safe fun. Westpac Stadium is much easier to get to for a visitor without having a car than many other grounds around the Australian mainland. if we had a few hundred of hardy souls flying over to the Cake Tin every second weekend there would be more colour and interest as well.