In his second attempt at the Gutbuster, Williamson set a record time of 2hr 48min 23sec, breaking the record set by Mark Williams, of Queenstown, of 2hr 49min in 2011. In his first try, four years ago, Williamson finished fifth.
''It's been a beautiful day for it - perfect conditions with a tailwind most of the way, up the climbs,'' Williamson said after Saturday's race.
The 75.4km race begins at Garston, 315m above sea level, and competitors ride through the Nevis Valley to Bannockburn. The highest point on the course is Duffers Saddle in the Nevis, 1300m above sea level, and riders have to tackle about 25 fords.
Williamson (25) is back living and working in his home town of Alexandra and focusing on the national road championships in Christchurch in early January.
''This is the start of my proper training and I'm pleased with the time. My main aim was really just to look after myself and get through it without any injuries.''
Williamson is working at Rock & Road Cycles in Alexandra, planning his wedding to fiancee Brydhi Ramage, studying for a commerce degree and also finding time to coach other cyclists.
''I've got a couple of young guys in the Gutbuster and a couple of older ones as well in it that I'm coaching and it's really good seeing them do well.
''I like helping other people achieve their goals - that's pretty rewarding, just as much as doing it yourself, really.''
While his focus was road cycling, he ''did a fair bit of mountain biking in the last year''.
''And it's good to be able to do races like this on your home turf.''
Gavin Mason, of Wanaka, was a distant second overall, in a time of 2hr 58min 32sec.
Erin Greene, of Dunedin, won the open women's category, finishing in 3hr 34min 25sec, ahead of Laura Flannery, of Dunedin, who came home in 3hr 50min 23sec.
It was the 15th year of the Gutbuster, which is organised by the Cromwell and Districts Promotion Group. About 200 riders competed.
The event is part of a summer series organised by the promotion group. The next races are the Bannockburn MTB Classic on December 30, the Lake Dunstan Triathlon/Duathlon on January 2 and the Lake Dunstan Cycle Challenge on January 11.
