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The 14-year-old was 0.24sec away from making the final of the 200m freestyle in Buenos Aires.
She finished in 2min 02.32sec to finish as the ninth-fastest of the 34 swimmers over five heats.
The mark was also narrowly short of her New Zealand age-group record in the event.
It was a particularly impressive result given Fairweather's age disadvantage.
The Kavanagh College pupil is the youngest competitor at the Games - her birthday is the December 31 2003 cut-off date.
That leaves her competing against swimmers up to 18-years-old.
Indeed, the winner of the 200m freestyle was 17-year-old Hungarian, Ajna Kesely, who swam at the 2016 Rio Olympics.
Kesely finished the final in 1min 57.88sec.