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The 15-year-old described herself as "pretty average" to begin with as well.
Having enjoyed the little competitions in swimming lessons from a young age, she decided to have a go at the bigger ones.
Now she has just returned from Korea and her first world championships.
On Monday, she leaves again for the junior world championships in Budapest, Hungary.
There she will compete in the 50m, 100m, 200m and 400m freestyle, as well as the 4x100m mixed freestyle and medley relays.
She hopes to make the finals, although she will just go in trying to do what she can.
Coming off racing the world's best of any age - where she notched two top 20 finishes - she has had some great preparation.
"It was an amazing experience," the year 11 Kavanagh College pupil said.
"Getting to race some of the big dogs was really exciting. Definitely a good learning experience for me."
Her rise has been rapid.
It was not until she shone at her first national age group championships in 2017 that she began to see herself as good.
Last year, she represented New Zealand in major age-grade competition for the first time.
In June, she claimed her first national open title - winning three golds.
Last week, she swam the fastest leg in qualifying New Zealand a spot in the 4x200m at next year's Olympics.
It had all happened quickly and she was a little unsure what to put her success down to.
"I don't know," she said.
"I put in a lot of hard work, so I guess what you put in is what you get out.
"I guess that's how I like to think of it, anyway."
She continues to take a quality over quantity training approach - seven swims per week and two gym sessions.
While that is plenty, there are swimmers out there doing up to 11 swims and three gym sessions.
For Fairweather it was a case of getting as much out of her sessions as possible.
It has clearly worked for her, although it was not always a case of the same formula working for everyone.
She is one to look effortless in the water.
Her technique has been refined, although she did feel she picked it reasonably quickly.
"I guess it was somewhat just there.
"But I've definitely had to tweak it bits to make it as good as it is."
The coaching duo of Lars Humer and Kurt Crosland have also had a big impact, not just on Fairweather, but a host of others.
Caitlin Deans is another in the Olympic mix.
Jessica Scott, Molly Law, Madison Wills and Jemma Wilson all had standout performances at the most recent NAGS.
That would suggest there could be more success down the line.
For now, Fairweather spearheads that.
Whether she will be in Tokyo for next year's Olympics remains to be seen.
She must be in with a good chance though.
Watch this space.