Some of the world's toughest cyclists are riding across the country with unimaginable speed and endurance as they compete in Australia's longest bike-packing race.

Seventy competitors from 16 countries set off last Saturday on the gruelling Indian Pacific Wheel Race, a 5,500-kilometre journey from Fremantle to Sydney.

It took just six days for race leader, Belgian cyclist Kristof Allegaert, to travel more than 2,800 kilometres to Adelaide.

The ultra-endurance cycling legend made the halfway point on Friday, taking a 20-minute pit stop in Adelaide's Victoria Square.

He was greeted by dozens of fans in awe of his stamina, including Adelaide Lord Mayor Martin Haese.

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"It's an incredible feat for someone to ride from Fremantle to Adelaide and to average up to 500 kilometres a day, which is quite extraordinary. I understand he's cycling for something like 20 hours a day," Mr Haese said.

Cycling promoter Angus Burrell said it was unfathomable just how many kilometres each rider was clocking up.

"Kristof just told me he's had a total sleep of six hours so far in six days. As a cyclist it's just super inspiring," he said.

In second and third place respectively was Britain's Mike Hall and Australia's Sarah Hammond, who were both well ahead of the pack.

Ms Hammond was the first female rider and the first Australian competitor to reach the halfway mark when she passed Adelaide.

South Australian riders, 46-year-old Davin Harding and 42-year-old Claire Stevens are also tracking well, travelling towards South Australia's Spencer Gulf region.

Cycling race could take weeks to complete

The fastest riders in the race will make the journey across Australia in about 12 days, but it could take weeks for the rest of the field.

Mr Burrell said many of the competitors are not professional riders, rather regular cyclists looking for a challenge.

"They are very much regular cyclists, we've got journalists, doctors and lawyers competing who just cycle for leisure on the weekends," Mr Burrell said.

"They've signed up to do the race and some of them are just trying to do 100 kilometres a day — and that's their goal and it'll take them however many weeks.

"There's no prize money, there's no trophy, there's no honour board or anything like that.

"You get to the finish and you have that satisfaction having done something that's pretty amazing and not many people would get the chance to do," he said.