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Central Otago won the six-team intercentre event with 37 points and was followed by Dunedin 36, Southland 31, South Otago and South Canterbury 30 and North Otago 15.
Buchanan demonstrated her class by remaining unbeaten in her five singles games.
She won four and drew with Bronwyn Stevens (Southland) 22-22 in the time limit game.
Buchanan beat Mata Frankum (South Canterbury) 25-19, Brenda Hope (North Otago) 25-13, Sarah Scott (Dunedin) 25-23 and Eileen Grant (South Otago) 25-17.
She demonstrated her cool temperament under pressure when she beat New Zealand development squad member Scott in a closely-fought second-round game.
Her most important win came in the final round, against Grant, when Central Otago needed to win its four games to win the six-team event.
At the end of the three rounds on Saturday, Dunedin led by five points from Southland and Central Otago.
But Dunedin faltered on the second day and won only three of its eight games.
Before the final round, Southland had 31 points and led Dunedin by a point. South Otago was on 27 and Central Otago 25.
The Central Otago pair of Debra Lloyd and Margaret O'Connor had three wins. In the final round they beat South Otago's Christine King and Marilyn Paterson 25-17.
The triples team of Jeni Heazlewood, Jane Anderson and Carol Woods also had three wins. In the final round ,they beat Southland 21-15.
The four of Imela Holdom, Linley O'Callaghan, Ruth Grant and Andrea Dowman won its second game when it beat North Otago's Aileen Blyth, Maria McMahan, Mary Watson and Pat Robinson 18-14 in the final round.
The Dunedin triples combination of Kay Hammond, Jan Barclay and Jan Hall won their five games. The most significant win was the close 19-17 win over South Otago's Glennis Barclay, Avis Rishworth and Marion Woodhead.
The only other unbeaten team in the senior event was the South Otago four of Wynette McLachlan, Bronwyn Cowie, Ngaira Whitaker and Mary Collett, which won its five games.
Its most important win came in the second-last round when it beat Dunedin 15-12.
Dunedin won the development sevens competition with 31 points and was followed by Southland 27, Central Otago and South Canterbury 24, South Otago 22 and North Otago 6.
The only unbeaten team was the South Otago four of Zenda West, Sylvie Melvin, Elaine Thomson and Pat Clark that won four games and drew 13-13 with Dunedin.
Singles bowlers Lee Grigg (Dunedin) and Debbie Jackson (Central Otago) won four of their five games.