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The Dunedin 15-year-old has concluded her first aquatic world championships by leading New Zealand to Olympic qualification in Gwanju, Korea.
Her 1min 58.84sec first leg of the 4x200m relay was a personal-best, breaking her own New Zealand age-grade record in the process.
That leaves her just 2sec behind Lauren Boyle's open 200m record set at the 2014 Commonwealth Games.
It was also the fastest time of the four New Zealanders who finished 10th, securing an Olympic spot in the event.
Other members of the team were former Dunedin swimmer Carina Doyle, Eve Thomas and Chelsey Edwards.
That will surely improve Fairweather's ever-growing chances of being in Tokyo for the Games next year.
It capped a week in which the Neptune swimmer took her first steps on to the senior world stage.
She finished in the top 20 in both the 200m and 400m freestyle, competing against world record holder Kate Ledecky in her first race.
That came in the 400m, when her time of 4min 12.30sec was enough to finish eighth in her heat and 17th overall.
Two days later, her 1min 59.68sec time in the 200m placed her 19th.
Not bad for a girl who is halfway through year 11 at Kavanagh College.
Indeed, it was only a month ago she surprised herself by qualifying for the championships.
At the same time, she won her first open New Zealand title, which soon became three titles - in the 100m, 200m and 400m freestyle races.
Those followed a series of age-grade successes, including breaking Rebecca Perrott's 43-year-old 400m freestyle women's 15 years national record in April.
That performance, which doubled as a national age-group title, helped her gain qualification for next month's junior world championships in Budapest, Hungary.
Before that she had only had two tastes of major international competition - last year's Youth Olympics and junior Panpacific Championships.
In that context it has been a major rise.
To be the best in New Zealand at such a young age is one thing.
To be in the world's top 20 in a sport in which the majority of the world's sporting powers - the United States, China, Russia and Australia among others - take seriously is another.
The Olympic qualifying meet begins on March 31 next year. That will certainly be one to keep an eye on.