It'd be interesting to do something like season members following those seasons. I suspect those who turn up no matter what, will mean a weak relationship there.
This would show that while scoring a lot of goals and finishing higher up the table is a good temporary way to boost crowds, the best way to increase attendances in the long run is to try to build the core fan base who will turn up no matter what. A way to do this would be to do away with another short term solution of home away games, making season tickets cheaper (though elasticity would also have to be taken into account), focusing more marketing efforts into Wellington rather than pushing so hard for games outside of Wellington, and other similar such ideas.
Of course the figures could show that it is strongly related to winning/league position/goals scored, although based on this season, you'd think playing home away games doesn't help achieve this goal so to boost members and attendance next season, you should play as many games as you can in wellington this season - maybe to give weighting to this theory you could also compare results in Wellington vs results in other parts of nz.