Brisbane Roar v Melbourne Heart 9,869 from the 442Oz attendance thread which is very well maintained and used to update wiki I believe. http://au.fourfourtwo.com/forums/default.aspx?g=posts&m=1629895
This chart allows easy comparison of attendances in the A-League regular seasons even though the number of games played has varied from season to season. The four S8 bars are intended to show how the percentages varied throughout the season. One conclusion that can probably drawn is that although widely derided the Top 6 play off championship at the end of the season reduces the number of meaningless and therefore low attendance games (ie <5k) towards the end of the season.
The ousting of Gold Coast and to lesser extent Fury helped a lot. NZK also, though it's an old story.