In a photo finish, the four leaders sprinted over the line, crashing to the ground and scattering officials.
Mason was awarded the elite title in 1hr 52min 45.92sec, with Williams second in 1hr 52min 45.93sec.
Brent Miller and Nick Clark, both of Christchurch, were hot on their wheels, finishing in 1hr 52min 46.76sec and 1hr 52min 51.99sec respectively.
The four were a tight-knit bunch from the start of the 47km race, which crosses three high country stations from Glendhu Bay, in Wanaka, through Motatapu, Soho and Glencoe Stations.
The biking field had 2100 competitors in elite, sport and recreational sections.
Mason said the race was "always going to be a lottery" at the end.
"It was a very entertaining race. It was like Russian roulette in the rivers, deciding which line we were taking."
Top-ranked women's representative Kate Fluker set a record to win the elite open female section in 2hr 16min.
Floortje Draisma (Wanaka) was second in 2hr 20min 51sec and Canadian Megan Rose third in 2hr 23min 29sec.
Braden Currie, of Wanaka, set a course record in the Motatapu Xterra triathlon, finishing in 3hr 48min 5sec.
The half-ironman distance attracted 106 individuals and seven teams, a drawcard being qualification for the world Xterra championships in Hawaii in October.
The race comprised a 2km swim in Glendhu Bay, a 47km mountain bike and a 15km run over the Miner's Trail course.
Fleur Lattimore (Nelson) took top honours in the women's section in 4hr 45min 19sec. Catherine Dunn (Wellington) was second in 5hr 9min 36sec and Sarah Bryant (Dunedin) third in 5hr 12min 56sec.
A multisport race comprised a 15km paddle on the Matukituki River, a 47km mountain bike and a 15km run on the Miner's Trail.
It attracted 16 entries and was won by Haydn Key (Queenstown) in 5hr 31min 6sec, ahead of Shaun Portegys, of Alexandra, (5hr 32min 5sec). Sia Svendsen (Christchurch) won the women's section in 6hr 8min 8sec.
The 42.2km off-road marathon was won by Cantabrian Blair McWhirter in 2hr 51min 41sec. Defending champion Vajin Armstrong, of Christchurch, was second in 2hr 57min 46sec.
A total of 538 runners and 65 walkers entered the marathon - the third-largest in New Zealand.
Victoria Beck, of Dunedin, won the women's section and finished third overall in 3hr 18min 13sec, setting a course record.
Sarah Coughlan (Queenstown) was second in 3hr 24min 34sec.
Baz Smith (Queenstown) and Jess Simson (Wanaka) won the 49km adventure run in 6hr 40min 3sec, beating the previous record (7hr 25min 6sec) set by Chris Dagg and Darren Blackhurst in 2010.
Twenty teams entered the race which has a four-peak traverse on exposed backcountry terrain, with a total descent of 2790m.
Mitchell Keir, of Ashburton, took honours in the Miner's Trail mountain run, finishing in 1hr 26min 9sec, heading a record field of 500 competitors.
Louisa Andrew, of Dunedin, won the women's title in 1hr 31min 57sec, the third consecutive year she has won the event, with Anna Yeoman, also of Dunedin, second in 1hr 32min 46sec.
