The Orica-Scott) bolter extended his lead in the sprint classification as he finished ahead of road world champion Peter Sagan (Bora-Hansgrohe) and Niccolo Bonifazio (Bahrain Merida).
Race leader Richie Porte (BMC) was untroubled during the 144km stage from Glenelg to Victor Harbour to retain his 20-second lead over Gorka Izaguirre (Movistar), who crashed hard inside the final three kilometres, and Esteban Chaves (Orica-Scott).
Izaguirre finished the stage after the touch of wheels brought him down but suffered a loss of skin. His condition is yet to be determined.
The victory by Ewan was one he crafted on his own in a disjointed finish broken up by the Izaguirre crash. He and Sagan were slightly off the pace and without team support but both managed to weave their way to the front to contest the finish.
"Today was pretty tough backing up after yesterday," Ewan said. "I went pretty deep and it really took me three-quarters of the race to really get into it again and it didn't help that we had to ride back a super strong breakaway.
"Since we got on the laps we were going full gas so it was a tough day. Unfortunately, I lost a few of my team-mates on the downhill part because it got really messy."
"I had Luke Durbridge there with me in the last three or four kilometres and he positioned me perfectly in the end. "I was a bit far back in the last kilometre and just jumping around on wheels but got through in the end."
Sagan too was interrupted by the late crash but threaded his way through the field before pushing hard in the bunch sprint.
“I was ok, there was one very bad crash in the last 3km and then I was really lucky with my team that we didn’t crash because it was a really fast finish," Sagan said.
"I hope the guys who crashed are ok. I’m not in the best shape yet, but that’s also a good thing because the season is really long. You have to think positively and we still have time to get in good shape.”
The early part of the stage saw a four-man breakaway led by Thomas de Gent build a four-minute advantage over the peloton, with the Lotto-Soudal rider scooping up intermediate sprint and king of the mountain points before they were all reeled in with 6km left.