Casey McLean and Jahrome Hughes scored five minutes apart after the break as New Zealand pulled clear of Tonga and secured a World Cup final place.
The Kiwis won 34-10 in Newcastle on Saturday night, overcoming strong winds that gusted above 60km/h.
Kalyn Ponga returned from a broken arm and helped create New Zealand's opening try, setting up Briton Nikora after attacking around the ruck.
Tonga's Isaiya Katoa used the wind to keep his side competitive, and Jojo Fifita finished a sweeping move for their first points.
Fifita scored again after Tonga shifted left from a scrum, putting his team ahead 10-6 in the 32nd minute.
Dane Gagai's try just before halftime, after Lehi Hopoate failed to gather a Jahrome Hughes grubber under pressure from Ponga, gave New Zealand a 12-10 lead.
The wind then disrupted Tonga's kicking, while McLean scored in the 48th minute and Hughes added another five minutes later to make it 24-10.
Xavier Willison crossed in the 70th minute, and Charnze Nicoll-Klokstad collected Erin Clark's cross-field kick on the buzzer.
New Zealand's only injury concern for the final is prop Naufahu Whyte, who left early in the second half with an ankle problem and did not return.