Marcus Fraser and Joe Ogilvie share third at 5-under-par 66. They were followed by Zoran Zorkic and Barry Cheesman at 4-under-par 67.
Moseley started at the 10th at Kooyonga Golf Club and picked up his first birdie of the day at the par-4 12th. He then birdied the 17th to make the turn at minus-2 before a torrid inward half.
The Australian hit his third shot to six feet for birdie at the par-5 first and followed with a birdie at the par-4 second. At the fourth, Moseley recovered from an errant tee shot and dropped his approach within one foot up the cup for birdie.
Moseley birdied the par-4 fifth to move to 6-under but stumbled with a bogey at the par-4 eighth. The 30-year-old bounced back at the very next hole, however. He reached the green in two at the par-5 ninth and two- putted for birdie to hold a share of the lead after the opening round of the first Nationwide Tour event of 2003.
'I got off to a bit of a slow start,' said Moseley. 'In years gone by, I never really gave myself a chance to see if I could grind out a good round but I can do that now.'
Thatcher parred his first four holes before a birdie at the fifth. He birdied the ninth to reach 2-under and collected a pair of birdies starting at the par-4 11th.
The 25-year-old, who won last year's Monterey Peninsula Classic, added birdies at 15 and 18 to complete a bogey-free round.
Peter O'Malley had six birdies on Thursday but his fine play was overshadowed by a blemish at the par-4 eighth. O'Malley triple-bogeyed the hole to finished with a round of 68. He was joined by Richard Ball, Pete Jordan and Fran Quinn in a tie for seventh.
Kim Felton, Peter Fowler, Brett Rumford, Chad Lydiatt, Andrew McLardy, Scott Petersen, Kyle Thompson and Todd Rose finished tied for 11th at 2-under-par 69.
This event is co-sanctioned by the Nationwide Tour and the PGA Tour of Australasia.