If you can get together a team of 3 (plus 1 substitute, I think) and you're willing to travel to Ballarat, the Royal South Street Society holds debating competitions every year.
http://www.royalsouthstreet.com.au/prizelist-> click on "debating" -> scroll down to pages 21/22 in the PDF file for information
From the PDF:
Section 821 Years 9/10School: Winning team $200, the Mars Debating Challenge Cup and a replica,
Runner-up team $100 and a certificate.
Individual: Winning team $20 each, and a medallion. Runner-up $10 each and a medallion.
Best Debater in Section heats: First - $60, Second - $50, Third - $40, Fourth - $30.
Each place getters will also receive a certificate.
Section 822 Years 11/12School: Winning team $250, the Mars Debating Challenge Cup and a replica,
Runner-up team $150.
Individual: Winning team $20 each, and a certificate, Runner-up $10 each and a certificate.
Best Debater in Section heats: First - $80, Second - $60, Third - $40, Fourth - $20.
Each place getter will also receive a certificate.
ENTRY FEES:School Teams $20.00 each.