Two Australian adventurers have completed their trek from the edge of Antarctica to the South Pole and back in 89 days on Jan 26, which also happened to be Australia Day.

James Castrission and Justin Jones cross the finish line at about 4 a.m, Sydney time, on the 2300km expedition.

"Mission accomplished!!" the Herald Sun quoted them as tweeting at 7.15 a.m.

The milestone will make them the first people ever to complete the trek unassisted.

They did it just in the nick of time, with the last flight out of Antarctica for the season due to depart today.

In 2008, Castrission and Jones became the first people ever to paddle unassisted across the Tasman from Australia to New Zealand.

But expedition logistics manager Rebecca Riel said that challenge "was nothing" compared to their latest achievement.

"This has definitely been the hardest thing they've ever done. I think what they have achieved will all sink in when they get back but right now they're just focused on crossing that finish line," she said.

Horrendous conditions at the start of the expedition forced the pair to start rationing their food more than 50 days ago.

As a result, they've lost an estimated 55kg of body weight between them.

In his most recent audio diary entry online, Castrission described how they were so thin that their hips and backbones were digging into the snow each night.

The pair undertook the challenge to help raise funds for You Can, a charity that assists youths with cancer. (ANI)