In 2018 she competed at both the 2018 Summer Youth Olympics and the 2018 Junior Pan Pacific Games. 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.