Fairweather was born in Dunedin, She is of mixed European and Māori descent, affiliating with Ngāi Tahu iwi.[2] She attended Kavanagh College and was head girl in 2021.[3]
In 2018 she competed at both the 2018 Summer Youth Olympics and the 2018 Junior Pan Pacific Games.[4] In August 2019, Fairweather won the gold medal in the 200 metres freestyle at the World Junior Swimming Championships in Budapest, Hungary, breaking her own New Zealand age-group record with a time of 1:57.96. She finished fourth in the final of the 400 metres, again breaking her own national age-group record with a time of 4:08.78.[5]