The 16-year-old is expecting a baby early next year, the agency that represents her said.
The father is her boyfriend of three years, Bradley Hull, 19.
Ms Castle-Hughes - who was born in Australia but moved to New Zealand with her family when she was a youngster - played the Queen of Naboo in last year's Star Wars: Episode III - Revenge of the Sith.
She will be seen next as Mary in The Nativity Story, set for release in Australia in December.