And if there was only 3 clubs in the Sydney area, that's probably what would happen.
I don't know why people compare Brisbane to Sydney clubs. They are a 1 team town in a city with a population of over 2 million people. Actually only getting 30k to their games isn't a huge percentage of people really. They have 1 league, 1 union, 1 AFL team playing in winter months. Sydney has 9 league teams, plus a union team, plus 2 AFL teams. Sydney clubs averaging 10k to their games is the equivalent of 30k in Brisbane.
Now, I'm sure the question is how. And it's this. Sydney has about 4.5m people. Divide that by 9 clubs, each club gets about 500k fan base. So, 1/4 of the size of the potential fan base of Brisbane. For a Sydney side to average 20k, that's 4 % of their potential fan base. Brisbane get 1.5% of theirs going to games.