The Ardglass golfer, a European Tour invite, is three shots off the lead held by American Peter Uihlein.
Sharvin hit seven birdies but dropped three shots on par five holes, taking a double-bogey seven on the 15th.
Belfast's Michael Hoey finished on five over including two bogeys in the last three holes.
Ireland's Paul Dunne finished on one over.
Media playback is not supported on this device
Sharvin slipped from five to three under, but made amends with a two at the par three 17th in Australia.
In what is only his second tournament as a professional, the 23-year-old expressed delight at his impressive start.
"This being my first European Tour event, it's really nice to post a nice number and see myself up the leaderboard with names I'm usually watching on TV," he said.
"My main focus this year is going to be Challenge Tour events and I can't think too far ahead, but I know if I play well I can compete."
Cormac Sharvin made a superb start to his first European Tour event as a professional shooting a four-under par round of 68 at the Perth International.