Behind a standout performance from playmaker Tyla Nathan-Wong, who created the New Zealanders' first two tries and scored the third in the final at Langford, British Columbia on Sunday (Monday NZT), the Ferns continued their recent dominance over rivals Australia with a 21-17 victory.
The Ferns had led 14-10 at the end of a first half that featured two tries apiece. Skipper Sarah Hirini and Niall Williams both crossed handy to the posts for the New Zealanders, both running nice lines off Nathan-Wong, while Ellia Green and Emma Tonegato struck for the Australians from long range.
Player of the final Wong raced clear for the decisive score early in the second half when she ran a nice inside support line off Hirini and dashed 50 metres to take her side out to 21-10.
But the Aussies were not quite done. Green nabbed her second try with less than a minute remaining when she swooped on a loose pass from Williams to get them to within four.
However they could not regain possession from the restart and ran out of time as the New Zealanders held on for a sixth successive victory over their trans-Tasman rivals and seventh win on the bounce in cup finals.
"Every time we play Australia we have to play our best," said Hirini after the final. "I'm so proud of the girls the way they dug in and stayed in the game."
Added Nathan-Wong: "It's always a good battle with Australia and they showed up. But we went out and executed the game-plan and had each others' backs."
The New Zealanders celebrated their title with their now customary haka, much to the delight of the crowd who stayed on for the presentations.
Earlier New Zealand won through to the final with a dominant 26-12 semifinal victory over the USA sparked by a first career hat-trick for Kelly Brazier.
The Ferns side rolled past the Americans after star speedster Michaela Blyde made a surprise return from a corked thigh injury to play a key role.
The New Zealand cause was aided by a second-minute red card to the USA's Alev Kelter for a dangerous tackle on New Zealand playmaker Ruby Tui.
The Black Ferns, by that stage, had already opened out to a 7-0 lead when Blyde scored a sensational solo try from halfway after they snaffled ball from their own kickoff. Brazier crossed twice more before halftime for the New Zealanders – the second on the back of another big run from Blyde – as they opened out to a 19-5 lead at the break.
Naya Tapper had managed a sizzling first-half score for the Americans but they were always going to be up against it with just six players on the field.
Brazier completed her hat-trick early in the second spell and Kristi Kirshe wrapped up the scoring with a runaway 70m consolation effort for the Americans. It was the fifth straight appearance in the final in Canada for the Black Ferns.
Afterwards Brazier shrugged off her first trio of tries in one match.
"It's not really my job in the middle of the park," she said. "I prefer to see the speedsters on the wing go over. I just happened to be on the wing and finished it off, so I'll take that."
Brazier said the key against the Americans had been to maintain possession.
"We knew we had to hold on to ball and add the phases together. They're a hard side to beat off one phase, so we tried to go 3-4 phases and wait till it opened up. That was the key to our win."
Australia edged France 26-19 in the second seminal at at Westhills Stadium.
Emma Tonegato scored twice for the Australians and Samantha Treherne and Alicia Quirk also crossed as they held firm after establishing a 21-12 halftime advantage.
The New Zealanders had defeated Spain 17-7 in the quarterfinal to clinch a Tokyo Olympics berth.
The series leaders – missing injured star Blyde – celebrated their victory after a late, clinching try to teenager Dhys Faleafaga set up the semifinal against the USA who beat Canada 12-7.
Hirini said the team were delighted to cement their place at the Tokyo Olympics.
"That's been our goal since the start of the season," she said. "We wanted to do it early on and to do it here in Canada takes a lot pressure off.
"But we still want to carry on the season as we've started."
Tui gave the Kiwis an early lead with a try after a dart from the base of a ruck.
Spain struck back with a couple of promising attacks, but Brazier pulled off two try-saving tackles for the Black Ferns.
New Zealand extended their lead to 12-0 after individual brilliance by Nathan-Wong.
She dummied a defender and then cut back inside to scamper away and score and convert her own try.
New Zealand missed the pace of Blyde, who hobbled off with a leg injury against China in pool play, but their defence, led by Tui and Brazier, was outstanding until the Black Ferns fell off three tackles in the leadup to a try to Spain's Maria Garcia.
But Faleafaga struck in the final minute after the Black Ferns stretched the Spanish defence with Brazier making inroads down the left flank and keeping the ball alive with a neat offload.
New Zealand went through pool play unbeaten on Saturday (Sunday NZT).
The Kiwis were too good for Russia and China in their opening two matches on Sunday (NZT), winning 26-7 and 45-0 respectively. But they faced a much tougher match in their final pool game, only just getting past England,12-10.