Bunny finished one stroke clear of friend and fellow Australian Peter Wilson, who stands alone in second place after firing a 4-under 66 on the South Course at Club de golf de L'ile de Montreal.
Stephen Gangluff, Michael Harris and Rob Oppenheim share third place at 3- under-par 67. Peter Tomasulo and Lindsay Bernakevitch are one stroke further back in sixth place.
Bunny, who has never finished better than 14th on the Canadian Tour, started his round on the back nine and was 3 under around the turn after birdies on the 11th, 14th and 18th holes.
The 27-year-old Aussie moved to minus-4 with a birdie at the par-5 second, and after six straight pars he completed his bogey-free round with another birdie at the par-4 ninth.
'No bogeys, that is what is best about the round,' said Bunny, whose best tour finish came at the Barton Creek Classic last year. 'I was able to keep it straight all day, which is what you have to do out here.'
Wilson, who was Bunny's traveling partner for much of last year, stumbled to a pair of bogeys during his round -- both on par-4s. But the 27-year-old made up for those missteps with seven birdies on the day.
'I'll tell you, this course is on steroids. A good start is nice, but with a good round you can pass about 60 guys,' Wilson said. 'There is still a long way to go.'
European Ryder Cup member Thomas Levet of France, who is playing on a sponsor's exemption, stumbled to six bogeys and one double-bogey on the way to a 7-over-par 77.
'I was just off today,' said Levet. 'One shot I'd be 10 feet short, the next 10 feet long. It was not a good day, but that's golf.'
Current money leader Jaime Gomez, who played in Levet's group, sputtered to a 4-over-par 74. Defending champion Stephen Woodard also had a rough day -- he shot an opening-round 76.
This event is back for the first time in nearly 60 years, having last been contested in 1945 at the Islesmere Golf Club. The champion of that tournament was Byron Nelson.