Pacy Canadian Charity Williams stepped in off her left wing to score by the posts and level the final at 7-7 a minute before the break.
But Gayle Broughton restored New Zealand's seven-point lead with a good swerve and run in the dying seconds to give her side a 12-7 halftime advantage.
The Black Ferns Sevens dominated the second spell in and out of possession, scoring three tries while Canada were unable to create anything.
Nathan-Wong set up Niall Williams for the first and swarming New Zealand defence led to turnovers and tries for Theresa Fitzpatrick (set up by Kelly Brazier) and Alena Saili right on fulltime.
The Black Ferns Sevens had earlier finished pool play with a 26-12 victory over England, after trailing 12-7 at halftime.
That set up a semifinal against France, which New Zealand won 24-7 thanks to four tries from Stacey Fluhler.
The New Zealand men extended their winning run over Australia to eight games by winning the fifth-place playoff 24-7.
The highlights included Regan Ware running 90m for a try in the first half and Caleb Clarke producing some nifty footwork for a try in the second.
The All Blacks Sevens scored 12 points in each half, while Australia didn't dot down until almost two minutes into added time.
The Kiwi team missed out on the semifinals after losing 26-5 to Fiji in pool play on day one.
With four of 10 tournaments played, the All Blacks Sevens still lead with 76 points but South Africa have closed the gap to six points after finishing runners-up, losing last night's final 12-10 to Fiji. The win was Fiji's first in Sydney.
The men's World Series resumes in Los Angeles on February 28-March 1 and Vancouver on March 7-8, while the women have to wait for the Hong Kong Sevens on April 3-5.