As the peloton battled temperatures of around 40 degrees Celsius, race organisers opted to shave 26.5 km off the Unley-to-Lyndoch stage and the 22-year-old Ewan timed his sprint to perfection to claim the overall lead.
Dutchman Danny van Poppel (Team Sky) finished second as Ireland's Sam Bennett (Bora-Hansgrohe) rounded out the top three and the race's main drawcard, double world champion Peter Sagan (Bora-Hansgrohe), finished safely in the pack in the same time.
"Everyone felt the heat out there today and to have the race shortened a little bit is what we needed," Ewan said after winning the opening stage for a second year in a row.
"A lot of riders ran out of gas in the lead out in that last straight because there was a lot of headwind there," he added.
"Many riders misjudged it and it was a super messy sprint but It's going to be a really nice feeling to have the leaders jersey tomorrow."
The six-stage race around South Australia is the opening event on the UCI WorldTour calendar and continues with a 148.5 km run from Stirling to Paracombe on Wednesday, a day which should suit the climbers more than the sprinters.
The race concludes in Adelaide on Sunday.
(Writing by John O'Brien in Singapore; Editing by Amlan Chakraborty)
