Port Lincoln's Emma Manthorpe finished ahead of riders from around the world after riding nearly 350 kilometres along South Africa's east coast.
Ms Manthorpe was one of 14 riders who took on the Race the Wild Coast, which was set on a nearly 350-kilometre course between Port Edward and Kei Mouth.
After four days of riding and three horse changes, Ms Manthorpe crossed the line first in a time of 77 hours, 37 minutes and 57 seconds on Friday, about an hour and a half in front of South Africa's Rangna Roux in second.
Of the 14 riders, eight finished the full course.
Ms Manthorpe said she had wanted to compete in the toughest horse race in the world, which had been the 1000 kilometre Mongol Derby, which she entered in 2017.
This was the third time the race had been held since its inception in 2016, the second held last year after a break in 2017.
"It was this race where it was declared by a few previous Mongol Derby competitors that while the Mongol Derby was the longest race in the world, the Race the Wild Coast was the toughest," she said.
"I fully committed to Race the Wild Coast earlier this year after I was married, my wedding was over and I was looking for my next exciting activity."
Ms Manthorpe said her preparation for the race included running, cycling, yoga and riding frequently.
During her ride Ms Manthorpe had to reach 12 stations where the horses were vet checked.
Three were horse change stations where the first riders to arrive received their choice of horse for the next leg of the journey.
All the horses were provided by endurance stud Moolmanshoek, owned by South African endurance rider Weisman Nel.
Ms Manthorpe said the severity of the terrain was a shock and just walking up to the start line was a challenge.
She said she tried to break away from the pack numerous times on the second day but believed her big break came at a river crossing.
"Jamey, my first alliance, and I saw the current was quite strong so we decided to cross it higher up and let the current take us down, whereas I think the group behind us tried to cross it closer to the GPS crossing point and they ended up in a bit of trouble getting washed up into a rocky area," she said.
"I didn't actually know I was in a good position the whole race, it wasn't until I was a kilometre away from the finish line that I realised I was in a good position to win the race."
Ms Manthorpe said the win had not sunk in yet and the experience was "very surreal".
She said it was difficult to compare the race to the Mongol Derby as one was 1000km and had to be completed in 10 days, while this one was 350kms and had to be completed in five days.
"The Race the Wild Coast is a constantly changing race, there is no way to become familiar or comfortable in this race whereas I was able to adapt to the conditions of the Mongol Derby earlier in the race," she said.
Ms Manthorpe said she would not take part in the race again but would love to return as a volunteer.
She said the support from the community had been overwhelming and "with a full heart I thank them all dearly".