I had a look at the data for last year and I think that the Brisbane longshots is just a myth. Here are some figures for the 2001 year based on Tabcorp divs:

AVERAGE WINNING DIVIDEND:
Doomben $8.00
Eagle Farm $7.66
Flemington $10.61
Sandown $7.99
Randwick $8.50
Warwick Farm $7.87

WINNERS OF OVER $15
Doomben 12.7%
Eagle Farm 10.4%
Flemington 17%
Sandown 10.2%
Randwick 13.5%
Warwick Farm 12.7%


So the greatest number of longshots is actually at Flemington - Brisbane is pretty much in line with the Sydney and Sandown figures.