A rare surge by wing and crowd favourite Ben O’Donnell saw him crash over Tasman’s line at the 20-minute mark, only for it to be judged to have been held up.
The Mako continued to dominate possession and territory with metre-eating wing Timoci Tavatavanawai a constant menace in the first 40 minutes, breaking tackles and forcing the home team on the back foot.
It didn’t get any easier with Magpies No 8 and former Tasman player Hugh Renton sin-binned in the 22nd minute.
The sole Magpies points came courtesy of loosie Sam Smith, who touched down in the 33rd minute to see Mako lead 19-5 at halftime.
A second-half attack in the 52nd minute earned the home side a scrum on Tasman’s line, but the efforts were again thwarted by tight Mako defence.
Despite Hawke’s Bay rallying in the second half, Tasman extended their lead to 22-10 after slotting another penalty.
In the 62nd minute, O’Donnell scored a try which Lincoln McClutchie converted to close the gap to 22-17.
The 76th minute saw Mat Protheroe dot down to level the score at 22-22. Danny Toala converted to lead 24-22.
Then came heartbreak for the home team with a Mako penalty on fulltime, the kick seeing Tasman through with a 25-24 win.
Tasman, a 2006 merger of Nelson Bays and Marlborough in the expansion of the NPC, had never won the Shield. Marlborough held the Shield for one tenure in 1973.
The Mako’s first defence will be against Wellington in the coming round.