Northland opener Baxter Skudder, with an unbeaten century, and his partner Will Potter (36) laid the platform for victory with a 101-run stand.
Second-drop Cam Gordon made 68 and the visitors posted 296-9 in 50 overs.
Off-spinner Daniel Stewart took three for 43 off 10 overs for Poverty Bay. He held the ball back skilfully against a patient Northland line-up which batted with game-sense.
Poverty Bay were 120-8 when rain stopped play two balls into the 31st over. Bay captain Thorn Parkes made 63 in 69 balls from No.3.
Northland's B Peck took three wickets for six runs off four overs — superb figures. Four of the six runs he conceded were wides.
Potter's knock of 36 was his first at the tournament and Skudder batted at No.7 in the T20 games.
'Our boys took enough time to get themselves in but still batted with the positivity that T20 brings,' Northland coach Karl Treiber said.
'Thorn was in charge in the middle of Poverty Bay's innings. He provided our boys with a real test in terms of setting fields and utilising bowlers.
'Max Briant (1-57 off 10 overs) has work-ons but also has a point of difference, in his height, as a seam bowler for Poverty Bay. The foundations are there with the offie (Daniel Stewart), too. I think it's heartening that Poverty Bay won three T20s — they've done well.'
DAY 2 results — T20s
POOL A, Round 3:
Hamilton 140-5 in 19.3 overs def Northland 136-8 in 20 overs.
Waikato Valley 79-5 in 17 overs def Northern Districts Invitation 78-6 in 20 overs.
POOL B, Round 3:
Bay of Plenty Coastlands 140-3 in 16 overs def Counties-Manukau 114-5 in 16 overs.
Poverty Bay 114-8 in 20 overs (T Mitchell 41, J Faulkner 25; R Hall 3-16, S McLauchlan 2-25) def Bay of Plenty Lakelands 112-4 in 20 overs (C Ingram 38, V Patel 21, B Astwood 21; T Parkes 1-16, T Mitchell 1-22, C Pooley 1-22).
T20 finals
Bay of Plenty Coastlands 72-3 in 10.4 overs def Hamilton 71 all out in 16.3 overs.
Northland 138-8 in 20 overs def Counties-Manukau 108-8 in 20 overs.
Waikato Valley 89-2 in 14.3 overs def Bay of Plenty Lakelands 85-8 in 20 overs.
Poverty Bay 123-6 in 20 overs (T Parkes 55, D Situ 34; M Sachdev 2-10, C Jakicevich-Roberts 2-14, G Chaplow 2-28) def Northern Districts Invitation team 115-6 in 20 over (L Halligan 45, N Rai 28; O Taylor 2-26).
50-over knockout matches yesterday
Bay of Plenty Coastlands 69-2 in 10.5 overs def Bay of Plenty Lakelands 68 all out in 22.1 overs.
Waikato Valley 166 all out in 43.5 overs def Hamilton 137-8 in 44 overs on Duckworth-Lewis.
Northland 296-9 (B Skudder 100, C Gordon 68, B Potter 36; D Stewart 3-43) def Poverty Bay 120 all out in 30.2 overs on Duckworth-Lewis (T Parkes 63; B Peck 3-6).
Counties-Manukau 137-6 in 34 overs def Northern Districts Invitation team 182 all out in 48.3 overs on Duckworth-Lewis.
Draw for 50-over matches today —
Cup: Bay of Plenty Coastlands v Counties-Manukau; Waikato Valley v Northland.
Plate: Bay of Plenty Lakelands v Northern Districts Invitation team; Hamilton v Poverty Bay.