RNZ reports Tongan is the third-largest Pacific group in New Zealand with just over 60,000 people identifying as Tongan.
More than half of them were born in New Zealand.
New Zealand's Minister for Pacific Peoples Peseta Sam Lotu-Iiga said the number of people who speak Tongan has been growing, from just over 23,000 people in 2001 to nearly 32,000 in 2013.
He said it's important to promote the teaching and learning of Lea Faka-Tonga at home, at work and in everyday situations.