Futures Betting, page-20

The winnings from gambling will be taxable if they are income from a business activity. This depends on the particular facts and circumstances of each case. Key factors will probably be the regularity with which you gamble, whether you use some method or system, and whether you rely on the winnings as your principle source of income. Get your own tax/legal advice of course.

One of the leading cases on this issue is at:
http://law.ato.gov.au/atolaw/view.htm?dbwidetocone=%2205:LRP:Federal%20Court:1989:Brajkovich%20v.%20Commissioner%20of%20Taxation%20(Cth):Judgment%20by%20Full%20Court;%22