A languages expert is calling for a national debate on which languages children should learn under a new National Party policy to teach every child a second language.

AUT Associate Professor Sharon Harvey, the lead author of a 2013 Royal Society paper on a national languages policy, says National's $160 million promise to offer every primary school child a second language "has the potential to completely transform language learning".

"This is the largest injection of funding into the languages learning area ever," she said.

But she said the country now needed to debate exactly which languages should be taught, and why.

Prime Minister Bill English and Education Minister Nikki Kaye said on Sunday that "at least 10 priority languages for the programme will be set following consultation with communities, with Mandarin, French, Spanish, Japanese and Korean likely to be included, along with Te Reo and NZ Sign Language".

"It will be up to school boards to decide which languages will be taught in each school, but they will be required to offer at least one second language," Kaye said.

But Harvey said a national policy was needed to ensure that children could carry on with their second languages at high school and have them recognised and supported by the National Certificate of Educational Achievement (NCEA).

She said an Auckland Languages Strategy, endorsed by Auckland Council in 2015, suggested that all children should learn te reo Māori and one other language, as well as English.

The other language "could be their community language (like Somali or Samoan), a major trade language (like Mandarin or Japanese), or New Zealand's other official language, NZ Sign Language".

"In Victoria there is an availability of more than 50 languages in primary schools," she said.

"I'm not suggesting that, but I think we need to think hard about the multilingual repertoires that our kids bring into school and whether we advance those or drop them and start from scratch with a new language like Mandarin.

"It might be a matter of thinking what numbers [of native speakers] would warrant teaching a language. But Bill English is not even talking about the top 10 languages in Auckland. He doesn't mention Hindi at all, and that is the language of a large community and also a major trade language."

Hindi is not currently available for NCEA. Apart from English, the NZ Qualifications Authority provides NCEA resources for only 11 other languages: Chinese, Cook Island Māori, French, German, Japanese, Korean, Latin, NZ Sign Language, Samoan, Spanish and Te Reo Māori.

A spokeswoman said three other languages could also be taken in NCEA: Tongan, Indonesian and Vagahau Niue. The last two are available only in unit standards, so they can't count towards University Entrance and their students can't achieve merit or excellence awards.

Meanwhile Green MP Marama Davidson reiterated the Greens' policy to make te reo Māori compulsory in all in schools from Years 1 to 10.

Labour education spokesman Chris Hipkins, who said in February that he personally supported all children learning te reo, said National's $60 million costing for a second language in all primary schools didn't "add up".

"Such a policy would require a language teacher in every primary school, or at least the equivalent resourcing across the country," he said.

"Assuming a salary of $60,000 and not accounting for any overheads or training, it would cost $117 million per year to ensure every primary school had a fully qualified language teacher."

He said English was asked on Sunday whether a school could opt not to offer a second language and replied," Well that could happen."

"Is National costing the uptake of this policy at just a third of schools?" Hipkins asked.

He said Labour would increase education spending by $4 billion over four years. The party's education manifesto promises to "ensure that all early childhood and primary school teachers are provided with an opportunity to undertake lessons in te reo Māori" and to "re-establish support for Pacific languages including reinstating funding for the Tupu series".