Was thinking about the issue in the advent of the 2008 NBL finals. Although the finals attracted some media interest in Sydney and the sold out, due to the Kings involved, the main headlines were the Kings and Bullets being up for sale.
Some people have said the switch to summer in 1998-1999 coincided with the decline in crowds. The NBL probably had to move due to competition from RL/RU/AFL. Nevertheless, basketball is going to face the challenge of getting people to watch games indoors in summer.
Here's a radical suggestion, but why don't the NBL look at more teams playing in open air arenas if/when the weather is fine? More people might want to go to games if they weren't cooped up indoors. I concede the weather is unpredictable, and Australia has had two very wet summers, but there are some multi purpose venues that could work.
In Melbourne, Rod Laver Arena is the perfect example of a venue that could host open air NBL, and has a retractable roof which could close in the rain. On a warm night, or even sunny afternoon, NBL could take advantage of that. The Arena attracted 14000 people in 1992. Are there other venues aroun the country that could host outfoor games?
OTOH should NBL look at having short outdoor weekend tournaments during summer, be it 5 man, half court or 3-on-3, involving all the NBL teams? These could be short, sharp and exciting, work a bit like Rugby 7s, 20-20 cricket, Beac Footy or Beach Cricket, attract people that usually don't follow basketball.