Māori captain Shima Fletcher won the toss and opted to bat on a lively wicket. The visitors were 3-6 when first-drop Josh Waller negotiated a tough delivery from opening bowler Jonah Reynolds (1-28-6-1) and the game was moved.
Sixth man in Kaden Cossill topscored for his side with a gritty 31 while Grant made 17 down the order.
Caleb Taewa, opening the bowling, picked up 2-7 off five overs, including two maidens.
Riker Rolls got 2-16-5 and Nathaniel Fearnley 2-14-4.1.
ND Māori made early inroads, but Poverty Bay captain Taye McGuinness stepped up with a vital 40 from 61 balls at No 5 before the late drama.
Orthodox spinner Louie Austin 2-45-10-1 had the best figures for the Māori while De Bruin bowled two magnificent spells.
The star performance of the day came from Counties Manukau opening batsman Sterling Speedy, who carried his bat to score a magnificent century, then took four wickets without conceding a run in a 99-run win over Bay of Plenty Lakelands.
The 17-year-old Speedy scored 107 off 117 balls, including 14 fours, as Counties-Manukau made 220-7.
He struck a pull shot for four to get off the mark, ran a quick single to mid-off to reach 50 and struck an on-drive for two to bring up his ton.
Speedy held a catch at first slip to remove dangerous Lakelands opener Kaiden Young (7) then, as the seventh bowler used, took 4-0-1.3 overs as Lakelands were dismissed for 121.
Scoreboard
Round 3, 50-over games
Poverty Bay 128-9 in 40.1 overs (Taye McGuinness 40 - Jack Grant 2 wickets for 29 runs off 10 overs, Louie Austin 2-45-10-1 (maiden), Caleb De Bruin 1-16-8-2) def Northern Districts Māori 127 all-out in 38.1 overs (Kaden Cossill 31: Caleb Taewa 2-7-5-2, Nathaniel Fearnley 2-14-4.1 overs, Riker Rolls 2-16-5) by 1 wicket.
Bay of Plenty Coastlands 88-6 in 27 overs (Nihar Parmar 20, Sam Cross 18; Oliver Penfold 2-8-4-1) def Hamilton 87 all-out in 30 overs (Ishi Singh 17, Jackson Botherway 13; Nihar Parmar 3-16-8-1, Matthew Dow 3-19-7-2) by 4 wickets.
Counties Manukau 220-7 in 44 overs (Sterling Speedy 107 not out, Akaldeep Lahory 65; Lachlan Hogan 2-15-6-1, Riley Kusabs 2-50-9-1) def Bay of Plenty Lakelands 121 all-out in 31.3 overs (Lucas Neill 26, Riley Kusabs 26, Logan Hutchings 25no; Sterling Speedy 4-0-1.3-1, Chainvir Singh 3-25-6-1, Daniel Chitty 2-21-6) by 99 runs.
Waikato Valley 240-7 (Josh Hart 63, Finn Bell 33, Joe Lang 31, Logan Hickey 27no, Jacob Hagoort 27; Tommy Litchfield 2-27-10-2, Owen Prinsloo 2-31-7) def Northland 150 all-out in 41.5 overs (Rafe Fleming-Bone 36, Josh Laing 30, Campbell Soanes 26; Josh Hart 2-17-4.5 overs, Joe Lang 2-21-6-1, Jacob Hagoort 2-32-7-1) by 90 runs.