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The 17-year-old Kavanagh College pupil delivered a brilliant Games debut in Tokyo last night.
She clocked a time time of 4min 02.28sec, qualifying fourth for the final of the women's 400m freestyle.
That final is scheduled for 2.20pm today (New Zealand time).
While a medal would have seemed unlikely prior to her heat - just making the Olympics was a big deal to her - Fairweather must now be seen as a genuine chance.
Prior to departing her goal had been to ''put down a solid performance''.
She did that and more - the amazement on her face upon finishing perhaps showed that more than anything else.
It was a race which took nearly four seconds off her personal best.
It also beat Lauren Boyle's New Zealand record in the event.
Fairweather took off at a fast pace and found herself in second - behind Australian gold medal favourite Ariarne Titmus - most of the way.
She briefly fell into third at the 300m mark.
But she did not fade.
Fairweather pushed hard in the final 100m, finishing just 0.66sec behind Titmus.
In other heats only American Katie Ledecky and China's Bingjie Li clocked faster times, of 4min 0.45sec and 4min 01.57sec respectively.
Fairweather, who swims for Neptune, was the 2019 junior world champion in the 200m freestyle.
She will also compete in the 200m at 10.05pm tonight.
Later in the week she will compete as part of New Zealand's 4x200m freestyle relay team.