Thorburn drew on the athletic skills of Costley, a holder of no fewer than 53 national medals, six of which are cross-country gold, when confronting the extremely wet and muddy course conditions.
"It was a case of changing my technique to short snappy strides," Thorburn said after his victory.
Thorburn thought the new course layout at the Waikouaiti racecourse was very similar to the one that will host the national event at Waikanae in two weeks.
Thorburn, who missed last year's Otago cross-country season because of international mountain-running commitments, was favoured to win the Edmond Cup last month, but received a huge wake-up call when defeated by Bevan Stevens (Caversham).
Saturday's race brought the two back in another head-to-head battle, and both were immediately embroiled in a four-way battle at the front of the field with Callan Moody (Ariki) and Tony Payne (Caversham).
With half the race gone, Thorburn decided it was time to make his move.
Heading into the final of the six 2km laps, he held a 15sec lead over a chasing Stevens, who had dropped Moody and Payne while setting off in pursuit of Thorburn.
Although gaining on Thorburn in the run to the finish, the margin was too great for Stevens as Thorburn crossed to complete the 12km course in 40min 33sec, with Stevens 8sec back in second and Moody third in 41min 16sec.
Thorburn was thrilled to be back in the winner's circle, having targeted 40min 30sec for the race.
"I've been training hard since Wingatui," he said of his defeat in the Edmond Cup.
"I gave my old coach, Bruce Milne, a call and feel I'm back on track."
There was an upset of sorts in the senior women's race when North Island champion Rachel Kingsford (Hill City) put an end to Shireen Crumpton's undefeated record.
Crumpton (Hill City) had run in the event 10 times since 1998 and had won each time.
Kingsford attacked from the start of the 8km event, opening up an early 12sec lead and gradually extending this to hold a 39sec advantage entering the final 2000m lap.
She crossed in 29min 38sec, with Crumpton 57sec back in second and Edmond Cup champion Kirsty Morris (Leith) third in 31min 18sec.
"I wanted to have a good blowout before the nationals," Kingsford said afterwards.
The junior men's race over 8km turned up a possible future champion in John Schreuder (Caversham), who overcame an early challenge from Alex Gorrie (Hill City) to win the title by 22sec from Gorrie, and record an impressive 27min 47sec.
Third was Ewoud Noordeloos (Nelson) in 28min 20sec.
The run from Shauna Pali (Hill City) to clinch the junior women's title over 6km was another standout performance from the championships.
Pali overcame the threat posed by Grace Fursdon, a member of the champion Otago University relay team, to win the title in 23min 56sec, with Fursdon second in 24min 40sec and Katrina Andrews (Hill City) third in 26min 41sec.
