The countdown is on for 225 runners keen to sweat it out in the 38km Routeburn Classic adventure run to be held on Saturday May 9 in one of New Zealand's World Heritage areas.

Runners will race from one end to the other of the Routeburn Track, a high alpine track traversing Mount Aspiring National Park and Fiordland National Park.

Now in its seventh year, entries to this year's race were snapped up online within 30 minutes of registration opening, and there are more than 100 athletes on the waiting list.

Runners will converge at The Divide, near Milford Sound, for the 8am start gun, and finish near Glenorchy. The fastest elite athletes are expected to cross the finish line in just under three hours.

Event director Evan McWhirter of Good Times Events said the race had gained a loyal following over the years, with good reason.

"It really is the most breath taking adventure run in the country, and arguably the world," he said.

"It generates worldwide interest and has significant appeal to both domestic and international athletes. Numbers are limited to meet safety requirements but it's become the source of running legend as previous competitors have spread their race stories around the globe.

"Many of those on the waiting list have a reasonable chance of getting in as athletes pull out because of injury, so it pays to keep up the training schedule and keep fingers crossed!"

New sponsor 2XU (pronounced Two Times You), an Australian-based sports apparel company, has joined the event sponsorship lineup and is offering all competitors and support crew the chance to buy a highly technical long-sleeved runners shirt at less than a third of the recommended retail price.