Shaun Matheson, of Riverton was third, and Ringakaha Paewai, of Gore, was third, two days after winning the Canterbury Circuit final in Christchurch.
Roberts finished last season as No 9 ranked open shearer in New Zealand, his nine finals reaping his second open win, at Cheviot, and second placings at the Ashburton and Reefton shows, and in A-grade competition at the Royal Easter Show in Auckland.
He's continued the form this season, with fifth placing in the New Zealand Merino final, starting the season in Alexandra, and second again at Ashburton, the events both being won by New Zealand representative and 2017 World Teams Champion shearer Nathan Stratford, of Invercargill.
He controlled Saturday's final shearing the 11 sheep in 12min 0.08sec, beating second-man-off Matheson by 13 seconds, and posting best points in both beard and pens judging.
Lionel Taumata, from Taumarunui and based in Gore, won the senior final, his fourth win in eight finals this season and the 10th win of his senior career, also after finishing third at Christchurch the previous day. He won by more than four points from runner-up and Chilean shearer Luis Pincol, who had scored his 14th senior win at Pleasant Point a fortnight earlier.
Finals newcomer Stacey Whitu, of Roxburgh, became the sixth intermediate among the seven finals in the grade in the South Island this season, beating runner-up, Gore shearer and Ellesmere show winner Brayden Clifford, who was shearing in his sixth intermediate final of the season.
RESULTS from the West Otago A and P Show shearing championships at Tapanui on Saturday, November 17, 2018:
Open final (11 sheep): Brett Roberts (Mataura) 12min 0.08sec, 41.4584pts, 1; Andy Mainland (Kapuka) 12min 44.47sec, 44.0416pts, 2; Shaun Matheson (Riverton) 12min 13.34sec, 45.3033pts, 3; Ringakaha Paewai (Gore) 12min 24.38sec, 46.219pts, 4.
Senior final (6 sheep): Lionel Taumata (Taumarunui/Gore) 17min 18.53sec, 28.593pts, 1; Luis Pincol (Chile/Geraldine) 7min 45.09sec, 32.665pts, 2; Ray Te Whata (Mossburn) 8min 41.59sec, 32.746pts, 3; Mitchell Murray (Amberley) 8min 24.09sec, 32.2045pts, 4.
Intermediate final (5 sheep): Stacey Whitu (Roxburgh) 7min 9.84sec, 33.892pts, 1; Brayden Clifford (Gore) 9min 7.12sec, 35.756pts, 2; Brodie Horrell (Gore) 8min 6.46sec, 36.923pts, 3; Wayne Malcolm (Winton) 9min 17.37sec, 37.4685pts, 4.