New Zealand is one of only three countries in the world where the non-religious are expected to outnumber the religious by 2050, according to new research.

New Zealand will join France and the Netherlands as having the religiously unaffiliated - referring to atheists, agnostics and those who do not identify with a religion - as the majority group in 35 years.

In contrast, most other countries in the world will see an increase in Christianity and a sharp rise in Muslim populations.

Islam is the world's fastest growing religion, according to the Pew Research Centre, with Muslims expected to make up 30 per cent of the world's population by 2050 - near parity with Christians, at 31 per cent of the global population - for the first time in history.

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New Zealand is one of eight countries where the Christian majority is expected to drop below 50 per cent of the population - alongside Australia, Benin, Bosnia-Herzegovina, France, the Netherlands, the Republic of Macedonia and the United Kingdom.

By 2050, Pew believes 45.1 per cent of New Zealand's population will be unaffiliated - making it the largest group in the country. Only France (44.1 per cent) and the Netherlands (49.1 per cent) saw a similar projection, the centre said in its Future of World Religions study.

But Dr Nick Thompson, lecturer in theology at the University of Auckland, believed this was a conservative estimate.

"I think it will be over 50 per cent well before the 2050 figure that the Pew people give," he said.

In the last two censuses, he said, there had been a more than 10 per cent jump in the number of people who identified as non-religious from 2006 to 2013. "It's pretty clear."

The only factor that would slow that increase would be immigration from countries where religious affiliation is a lot higher, Dr Thompson said.

"Especially among Pakeha, Maori and even to an extent Pacific Island communities, religious affiliation is either in decline or slowing down."

New Zealand had been "muddling along on a kind of cultural Christianity" since the '60s, Dr Thompson said.

"It may be that Christianity continues in the culture a long time after it's declined as a religion.

"When you think of Christmas, for example, it's a really, really popular festival, even though it would be hard to claim that the majority of people observing it were particularly religious.

"So the way we observe Christmas now may give you a sneak preview of the way we treat Christianity at the end of this century."

Mark Honeychurch, president of the Humanist Society of New Zealand, said the Pew projection was "not overly surprising. The only thing I was surprised at was that it would be up until 2050."