Duffy and Great Britain's Jessica Learmonth took control of the race on the bike leg but broke away after five kilometres to post a time of 56min 50sec. Canada's Canada’s Joanna Brown was third.
"Jess was phenomenal. I held onto her, we work well together, we race so often together that we have an understanding," Duffy said.
"The Games don’t come around often and it’s hard to get into. I am from a tiny country with not a lot of medallists so I am expected to win. I try to forget about it because I do it every week, so that’s how I deal with the pressure."
The best placed Australian was Ashleigh Gentle who finished fifth after dropping off the pace set by the chase group. Gillian Backhouse finished ninth and Charlotte McShane 11th.
Duffy finished the swim leg behind Learmonth and the pair battled to a stalemate at the end of the bike leg.
