Sian Williams, who came in at 4 hours and 57 minutes, beat her male opponents by 18 minutes, making her the second female winner in more than two decades.
The 29-year-old said she had started training seriously only six weeks ago.
“I felt a bit lonely all by myself for most of the race. I was nervous about the unexpected and how my body would pull up,” she said.
Seven hundred competitors turned out at Leighton Beach in the early hours of the morning to swim the longest open water swim in Australia and one of the longest in the southern hemisphere.
Dalkeith resident Benjamin Hewitt and City Beach resident Jamie Bowler made up the first mixed duo to finish, while Cottesloe’s Gary Claydon was part of the first male team of four to cross the line.
The first female team of four to finish included City Beach resident Sally Flavel.
Nedlands resident Paul Luscombe was part of the winning mixed team of four.
Three Cottesloe residents and one Nedlands resident (Anna Eisenhauer, Patricia Burton, Nicola Corsbie, Brigid Tait) made up the all-female Claremont Masters team of six who were the first over the finish line.
Millie Martino, of Nedlands, was part of the first mixed team of six to finish.
Hamersley resident Heather Snowdon and Wembley resident Kira Flanagan were the first female duo to finish.