After a bye in the first round and a rain-interrupted tie, Harbour fielded a top-strength squad.
Balmacewen, regarded as co-favourite with Harbour, lost by a set on countback to Mornington-Roslyn Black after several long encounters.
Balmacewen was missing top man Conrad Wong who has exam commitments.
The key moment for Harbour came when Campbell Higgins, a student from Tauranga, outlasted professional coach Shaun Paringitai 6-7, 6-0, 6-2 thanks to superior match fitness.
Veteran Phil Mirfin and Balclutha junior Ryan Eggers contributed with two good wins.
Mornington-Roslyn Black was led by No 1 Southlander Oliver Cuthill, a former US scholarship holder, who defeated Hee Bum Chung 6-4 6-2.
Rob McGavin battled to two three-set wins in the lower order for Balmacewen.
The women's competition began with close results and some new faces.
Titleholder St Clair beat Balmacewen 2-1 but newcomer Pauline Hobbhahn, a German exchange student, beat Lauren Fowler 7-6, 6-1 before the dominance of Sian English, a nationally ranked junior, made the difference.
Eastern Harbour beat the combined Harbour-Southern Country team 2-1 with the surprise result coming from Hanna English (13), who beat her older and higher-rated rival, Georgia Hume, 6-3 6-4.
Hume had changed clubs to play in the No 1 position and has work to do to hold out team-mate Debbie Cartwright, a former Southern representative, who beat Southlander Courtney O'Callaghan 6-1, 6-1.
Hume and Cartwright won the doubles 7-5, 6-3 to clinch the tie.
