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The pair attend Queenstown Primary School and train at the Queenstown Events Centre's Alpine Aqualand and the community pool at Wakatipu High School.
Molly said since living in Bali about two and a-half years ago, where their father had been a project manager, swimming had become a daily occurrence.
At the last national competition gained four top 10 placings.
She is looking forward to attending a camp next month for the country's top swimmers.
Being such a high achiever in swimming made her ''feel good, but sometimes I feel a bit mean, in a way, because I take all the trophies''.
Sister Ashlea swam in Bali, too, and by the next national competition in February she will be old enough to enter, having already reached the qualifying time.
Both said they enjoyed swimming because it was fun, it was fulfilling to reach achievements, and they made friends from throughout New Zealand.