Australian triathlete Penny Slater has had the biggest result of her career, finishing seventh at the Ironman-distance world championships in Nice.

They 28-year-old from Canberra overcame a significant mid-year injury and vomiting during the marathon leg to secure her first top-10 finish at the worlds.

It's also the first Australian top-10 finish at the Ironman world championships since Sarah Crowley was seventh at Hawaii two years ago.

Slater has endured a tough year - she was second at Ironman Texas in late April, but then needed eight weeks off running because of a leg stress fracture.

"I still can't quite believe it, I knew that I was a bit underdone going into this, just because of having the injury ... but I just held it together for that marathon as best I could," Slater said.

"I started vomiting and fell apart a bit but got some coke into me and I was alright.

"From about 27km I was holding on for dear life ... you kind of just let yourself believe that I actually achieved the goal that I came here for.

"It's amazing to have my Mum and my partner here and heaps of Australians out on the course cheering for me, it was awesome."

German Laura Phillips won the 3.8km swim, 180km cycle and 42.2km run world championship in eight hours 45 minutes 15 seconds.

Britain's Kat Matthews was second in 8:53:20 and American Chelsea Sodaro, the 2022 champion, clocked 9:04:38.

Slater, who quit her job as a high school teacher two years ago to focus on long-course triathlon, finished in 9:21:47.

The next Australian was Kylie Simpson, who took 24th. Simpson posted the fourth-fastest run split, a blistering two hours 56 minutes 26 seconds.

British defending champion Lucy Charles-Barclay pulled out on race eve because of a leg injury.

Then German star Anne Haug, who has been on every Ironman world championship podium since 2018, had a mechanical problem early in the bike leg and also withdrew.

Slater was 25th out of the water from the swim, which she said was good for her.

She then thrived on the hilly bike course, reaping the benefits of having been in Nice for six weeks before the race to prepare.

"My coach just said to me before the start that my job is just to chase all day and never give up and I was thinking that, I actually really enjoyed the bike course," she said

Slater was also the top Australian when the women's world championships were held on the famous Kona, Hawaii course last year, finishing 13th.

The world championships have become too big for Kona, forcing organisers to split the men's and women's races.

For the first time last year, the men raced in Nice and the women in Hawaii, with the races switching each year.

The men's world championships will be held at Kona on October 26.