Fairweather lowered the eight-year-old mark which was held by another New Zealander - Lauren Boyle - by nearly three seconds to continue her superb form in 2023.
Pushed all the way by second-placed Australian Lani Pallister, the 19-year-old pulled clear over the final two lengths to win in 4min 01.09s on Saturday (local time).
It is the fastest World Cup time since Boyle swam 4min 04.26s in Dubai in November 2015.
Fairweather's time is the fourth-fastest of her career, having joined the elite sub-four minute club in July at the world championships, where she clinched bronze in 3min 59.59s.
In the women's 200m freestyle on Sunday, Fairweather had to settle for the silver medal (1min 56.11s), beaten by Siobhan Haughey from Hong Kong in a time of 1min 55.10s. Pallister was third.
