Thorburn (29) finished third in the 12km senior men's race in 39min 42sec behind Alex Parlane (Auckland) in 39min 14sec and Melbourne-based Shaun Krawitz in 39min 29sec.
His effort helped Otago finish third in the six-man team race behind Wellington and Auckland.
The other scoring members of the Otago team were Bevan Stevens in 41min 11sec (12th), Tony Payne in 42min 06sec (18th), Peter Meffan in 43min (25th), Daniel Balchin in 43min 42sec (30th) and Jason Palmer in 43min 44sec (31st).
Thorburn, who runs for the Ariki club in Dunedin, is a converted duathlete who switched to running nine years ago when he joined Lin Rayner's squad.
He competed in duathlons in his home town of Christchurch before entering the Otago Medical School and was third in the two-day event at last year' Coast to Coast.
Thorburn, who has represented New Zealand at the world mountain running championships, proved he has the endurance to succeed in tough cross-country events.
Rachel Kingsford, who represented New Zealand at this year's world cross-country championships, was third in the senior women's race in 29min 59sec behind Fiona Crombie (Canterbury) in 29min 39sec and Lisa Robertson (Auckland) in 29min 53sec.
Megan Balchin was 13th and Barbara Roy 19th, but Otago did not have a fourth runner for the team race.
Maria Sleeman won the women's aged 50 to 54 title in 26min 49sec.
Shauna Paul was fourth in the junior women's race and John Schreuder sixth in the junior men's event.
Fourth placings were also gained by Anna Kean (women aged 16), Andrew Lonie (masters men aged 35 to 39) and Alan Nicholls (masters men aged 55 to 59).
