Robin Bishop, of Tauranga, who is leading a petition to make English an official language in New Zealand.

It's a secret on the tip of everyone's tongue: English is not an official language in New Zealand.

Te reo and sign both have Official Language status under law, but English does not. Although it is used by custom in the courts, Parliament and throughout the public sector, only the Tax Administration Act legally requires records to be kept in English.

When Robin Bishop, of Tauranga, overheard this fact in the street, she made it her mission to investigate.

"So I asked [Tauranga MP] Simon Bridges about it, and he said [he had] no idea. Then put his library experts on to it, and he said you're right, there's no legislation."

In February, Bishop launched a petition to make English an official language beside te reo and sign, starting the campaign with the signatures of four of her colleagues.

She hopes to gather more than 3000 signatures, after which Bridges will present it to a parliamentary select committee.

A spokesman for Justice Minister Amy Adams said English did not need legal recognition to be official.

"Te reo and sign language have statutory recognition, but English is a de facto official language by virtue of its widespread use," he said.

Labour disability issues spokeswoman Ruth Dyson, who was the member in charge of the New Zealand Sign Language Act 2006, said Maori and sign language needed special status because English was the dominant language.

"English was the only language in common use in broadcasting and law, so that's why sign language and Maori needed to go through the law change."

In order for the petition to be successful at select committee, it would need to be recognised as a worthwhile change, Dyson said.

"It's pretty clear to me that English is our primary mode of communication. So it will be quite difficult to make it go through.

"I can't see that there's a need for our language to be protected by law."

English is spoken by 95.9 per cent of people in New Zealand. The second most common language is Maori, spoken by 4.1 per cent of the population. Sign language is used by 24,090 people.

Victoria University professor of linguistics Miriam Meyerhoff said it was "a bit of a paradox" that English was not legally recognised as an official language.

"I am not aware of English being under threat. I think that's why people haven't seen this as being a priority. It's [been] more about protecting the indigenous languages."

Bishop's petition is on track to be complete in October.