Steffen has been unbeaten over the distance this year, and the title is also her 20th career 70.3 victory.
"I'm glad to come back here this year and get the win," she said. "I had a perfect day today, that doesn't happen very often."
Terenzo Bozzone chased hard all day and claimed third place in Mandurah, while Luke Bell finished in sixth.
Meanwhile, after three days of races staged across different formats, Javier Gomez emerged as champion of the inaugural Island House Triathlon in the Bahamas.
The Spaniard who has distinguished himself this year as the first person to achieve five ITU world titles added another trophy to his cabinet by logging the fastest combined times from an individual time-trial triathlon, draft-legal Olympic Enduro, and a non-drafting sprint distance race. He takes home $60,000 from the prize pot that pays ten deep.
"I gave my best and I'm sure the others did as well. We are professionals and when we get on a start line, we race hard," he said. "Can't be happier finishing the season this way -- victory at the Island House Triathlon! What an amazing and unique event!"
Gomez also won the fastest overall swim and run prizes. Fellow Bahrain Endurance athlete Ben Hoffman finished in sixth place overall, backing up well after his exploits at the XTERRA World Championship and the Ironman World Championship.
Sam Appleton's victory at Ironman 70.3 Austin puts an exclamation point on this weekend's racing. The young Australian was on the hunt out of the swim, led off the bike, and ran a sub 1:15 half-marathon to win by more than a minute. He adds this title to the two he has already won this year at Cairns and Vineman against the likes of two-time Ironman 70.3 world champion Craig Alexander, showing he is one to watch out for over the half distance.
Photo credit: Delly Carr / Bahrain ENDURANCE Media