While the Bay kept on winning, one of the talking points was on the other side of the draw as Auckland upset Wellington.
The team from the capital, who had a perfect winning record heading into yesterday's clash, lost by 3-2 at a crucial stage of the championship.
Tasman's dream of winning the event faded when they were also upset by Hawke's Bay (3-2) in round five.
While Wellington and Tasman lost their way, Otago also lost some of their momentum. Otago have a tough final day when they face hosts Waikato and then Bay of Plenty with those matches going to decide the semifinalists from Division Two.
Defending champions Manawatu-Wanganui, who were held by Northland to a half in round four, also remained undefeated when they overcame North Harbour 4-1.
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Toro NZ Men's Interprovincial
Leaderboard after Round 5: Division 1: 3.5 (team wins) 15 (individual wins) Manawatu Wanganui, 3 16 Wellington, 3 13 Northland, 2.5 11.5 Auckland, 2 10 Tasman, 2 10 North Harbour, 1 5.5 Hawke's Bay, 0 4 Poverty Bay East Coast; Division 2: 4 14.5 Bay of Plenty, 3.5 15 Otago, 3 11.5 Waikato, 2.5 10 Canterbury, 1 8.5 Southland, 0.5 9 Aorangi, 0.5 6.5 Taranak