The street race held in Adelaide is a prelude to the Santos Tour Down Under which is the first WorldTour race of the year.
Ewan (Lotto-Soudal) outsprinted three-time world champion Peter Sagan (BORA-hansgrohe), who won last year's edition, while Alex Edmondson (Mitchelton-Scott came in third.
Italian star Elia Viviani (Decurninck-QuickStep) was among the riders who crashed on the hectic last lap of the circuit race.
At 200 metres from the line, Ewan launched his low tuck sprint to held off Sagan and Edmondson to claim his third Down Under Classic.
“During the first part of the race I tried to hide safely in the bunch," Ewan said. "The last twenty minutes of the race, we moved to the front of the peloton.
"[Carl Fredrik] Hagen and [Thomas] De Gendt did a great job in controlling the early break and the other four Lotto Soudal riders kept me well-positioned at the front of the pack.
"We were lucky to avoid the big crash which happened just behind my back, so I didn’t see it myself.
"In the very end of the race, Roger Kluge had to close a little gap before I could launch my sprint. I felt that Sagan was on my wheel but I was confident that I could hold him off.”