Downes made a 15-foot putt on the par-5 17th after reaching the green in two with a driver and 7-iron. He finished with a 10-under 206 total on the Auckland Golf Club course.
''It was pretty gusty out there, and the greens got bumpy in the afternoon, so I'll definitely take a 69,'' said Downes, celebrating his 23rd birthday.
New Zealanders Mahal Pearce (69) and Gareth Paddison (70) were tied for second, and countryman Michael Campbell was three strokes back after a 67. Australia's Wayne Grady (67) and American Darrell Kestner (68) were 6-under.