Downes, in the fifth group out in a rainy Round 1, posted six birdies and one bogey for a 5-under-par 67 at Moonah Links on the Mornington Peninsula, which is hosting the prestigious event for the first time.
Its a good field and 67 is out there. I probably could have gone a little lower out there, but Ill definitely take 67, said the 23-year-old Queenslander.
Peter Lonard is alone in second place, one off the lead. Lonard made the turn in even par, but notched four birdies on the back for a round of 4-under 68.
Six players are tied for third at 3 under, including 2001 champion Stuart Appleby, Craig Parry, Richard Green, Craig Jones, David McKenzie and 2002 U.S. Amateur champion Ricky Barnes.
McKenzie held the tournament lead at 6 under through 15 holes, only to bogey each of his final three holes.
Applebys day started with four consecutive birdies out of the gates, before a bogey at the fifth. He birdied the eighth to again get to 3 under, and remained there as he offset a pair of bogeys with a pair of birdies on his second nine.
Other notable players include: 1999 and 2000 champion Aaron Baddeley (71); Australian PGA champion Peter Senior (71); Robert Allenby (72); defending champion Stephen Allan (72); reigning U.S. Amateur champion Nick Flanagan (77).
Allans day was erratic at best. He had six birdies, four bogeys and a double bogey.
Only two players have successfully defended their title in the last 26 years: Baddeley and Greg Norman (1996).