The 15-year-old Neptune swimmer was simply hoping for a personal best in the 15 years girls 400m freestyle final at the national age group championships in Wellington on Saturday.
Smashing Rebecca Perrott's 43-year-old New Zealand record in the grade - set at the 1976 Montreal Olympic Games - was a pleasant surprise.
It had been New Zealand swimming's longest-standing record.
Her previous personal best of 4min 13.55sec - the 14 years national record she also holds - was already below Perrott's 4min 14.7sec.
Now, Fairweather just needed to do it as a 15-year-old.
She did just that, touching the wall in 4min 12.10sec.
It also took her below the records in the three age groups above her.
The time was her 11th long-course national age group record, to go with six short-course records.
This one did have a little extra significance.
"It's definitely a really cool record to break - probably one of the best ones," Fairweather said.
It was not the only success the Kavanagh College pupil had at the weekend.
Fairweather finished the meeting with four national records - also breaking the 100m, 200m and 800m freestyle marks - and eight gold medals.
She also combined with Molly Law, Jessica Scott and Natalie Hutchens to win silver in the 16-and-over 400m freestyle relay.
"It was beyond what I was hoping for, so I'm very happy."
Her performances have earned her selection for the New Zealand team to attend August's junior world championships in Hungary.
Dunedin coach Lars Humer, who coaches Fairweather, was named as one of three coaches for the team.
Fairweather also claimed gold in the 200m medley. She was followed by fellow Dunedin swimmer Madison Wills (Kiwi) in second place.
In the 14 years 400m freestyle, Erika's sister Jade had was edged in a tight finish, placing fifth, 0.28sec off bronze.
