Australian Melissa Gorman copped a shattering disqualification which cost her a podium finish at the world open water swimming championships.

Gorman, the reigning 5km world champion, looked to have secured third place in a frantic bunched finish to the women's 10km race on the opening day at Roberval, Canada on Saturday.

However, following a protest, the Queenslander was disqualified, along with French swimmer Aurelie Muller, for unsportsmanlike conduct in reaching the finish line.

The 39-strong field had battled aggressively over the four-lap course, with five swimmers ultimately red-carded for over-vigorous action including Gorman, who had led at the 7.5km mark.

Italy's Martina Grimaldi won the race by 12 seconds, but the battle for the minor placings became a torrid jostle with countrywoman Giorgia Consiglio finally awarded second place and Fang Yanqiao of China third.

"Regardless of the result, I was really happy with the way Melissa swam the race," said Swimming Australia open water head coach Greg Towle.

"Everything went to plan, except for the finish I guess, but racing as strongly as Mel did is a really good sign that her 10km is coming along and things are going in the right direction for the London Olympics."

Australian debutant Stacey Hansford was 18th in her first international competition.

AAP