Ewan hit the front of the peloton with 100 metres to go with no rider able to stay on his wheel as he tucked in for the victory.
He finished ahead of fellow Australian Mark Renshaw (Dimension Data) and Wouter Wippert (Cannondale).
“I felt good all day and they (Orica-GreenEDGE) backed me right from the start,” Ewan said. “To win a road race you have to be very conservative during the day, the victory usually goes to the one who's got the freshest legs and the best team in the end."
Despite finishing second, Renshaw was happy with his performance against the Australian rider of the moment.
"The finish was very hard to judge with a head wind finish and slight uphill," he said. "I maybe went a little too early once I'd seen the finish banner and that cost me the chance to win.
"Ewan did a really nice sprint, timing it perfectly behind me, he is in top shape where I have bigger objectives coming soon in the season."
The day was warm and the peloton relaxed on what was a predicted sprint finish for the 130.8km run from Prospect to Lyndoch which ended with four laps of a street circuit.
Three man break containing Alexis Gougeard (AG2R), Martijn Keizer (LottoNL-Jumbo), and Australian Sean Lake (UniSA) went from the gun.
Lake was the last to resist, caught by the peloton with five kilometres to go.
“A long ride up the road suits me more than the other guys so it was my job to break away for UniSA,” Lake said.
He was rewarded for his efforts in the mountains classification after he crested the only categorised climb of the day ahead of Gougeard and Martijn Keizer.
“I’m absolutely stoked to get the King of of the Mountains jersey,” Lake said. “I gave it all in the climb.”
But the story of the race belonged to the first WorldTour stage winner of 2016 who continued to confirm his place as one of the fastest men in world cycling.
“It’s a really proud moment for me, I’ve never led a WorldTour race before and to do it for the first time in my home country is a real honour for me," Ewan said.
Ewan now leads the race heading into the 132km second stage from Unley to Stirling. He holds a four second general classification lead over Renshaw and Gougeard.