SportingBet Australia is offering odds on when the federal election will be held. Their opening prices are below, with the favourite being Nov 24.
Any Saturday in August/September $26.00
October 6th     $15.00
October 13th     $9.50
October 20th     $7.50
October 27th     $7.50
November 3rd    $7.50
November 10th    $5.00
November 17th    $5.50
November 24th    $4.00
December 1st     $4.50
December 8th    $6.50
December 15th    $9.00
December 22nd    $15.00
December 29th    $21.00
Any Saturday in January or beyond $13.00
Andrew Leigh's Archived Blog 2004-2010 
I can’t believe they’re only giving $7.50 for Derby Day (Nov 3)! It gets many more people than the AFL Grand Final so any government that called an election then would raise considerable ire.