A late try to tournament standout Portia Woodman wrapped up victory for New Zealand, who beat Australia for the first time in this series.
The Australians had won two trans-Tasman finals, in Dubai and Sao Paulo, while they were eliminated in the semi-finals of the Atlanta tournament, where New Zealand beat Canada in the final.
Series defending champions New Zealand have improved to 76 points, two more than Australia, while Canada are third on 64 heading into the fifth and final event in Amsterdam next month.
It was appropriate that speedster Woodman should have the final say with a 60m try.
It was her 12th of a tournament which New Zealand dominated.
They hammered Brazil, China and Spain in pool play, before accounting for France 42-7 in the quarter-finals and Fiji 26-0 in the semi-final, where Woodman bagged a hat-trick.
Australia provided the sternest test in wet final-day conditions, scoring tries through Emilee Cherry and Sharni Williams.
However, New Zealand took command through first-half tries to Sarah Goss and Huriana Manuel and another soon after the break to Carla Hohepa.
Earlier, Australia beat Spain 12-5 in the quarter-finals and Canada 5-0 in the semi-finals.