Growcott, who lives in Australia, was met by police when he came off a cruise in Wellington last year.
Judge Brian Callaghan said he had never sentenced someone of Growcott's age and allowed discounts for his ailing health, cooperation with police and early guilty pleas.
But the victim said his health was not as bad as it was made out to be.
She was about 12 when the sexual offending began and her sister was assaulted from when she was 7 until she was 10 years old.
The 38-year-old said the situation had torn apart her family and she was speaking out to encourage other victims to come forward.
"There will definitely be other victims. If he's done it to anyone else I guess there's still time for more charges, for more action," she said.
- Additional reporting: Jess McCarthy of Newstalk ZB