The cultural differences between the North and South Island are widening.

South Islanders are whiter, and less likely to speak a second language or have a formal qualification than their northern neighbours.

Census general manager Nancy McBeth said data released last week showed an increasing difference between the two islands, driven largely by different immigration patterns and population ageing.

McBeth said the difference was most marked in ethnicity and family make-up.

Figures showed 90 per cent of South Islanders identified themselves as a New Zealand European or as a New Zealander - a category introduced for the first time this year which has been largely interpreted as meaning Pakeha - compared with 70 per cent of North Islanders.

South Island households were more likely to be made up of couples living without children - the opposite to the North. McBeth said this was probably because South Islanders tended to be older and had children who had grown up and left home.

Auckland University demographer Ward Friesen said the ethnic make-up of smaller towns around the country had stayed largely the same since the last census in 2001, but in Auckland the proportion of Asians had increased from 11% to 19 per cent.

Auckland was now home to two-thirds of the country's Asian population and two-thirds of the people from the Pacific, he said.

For those seeking relief from the so-called man drought, things are marginally better in the South Island. For every 100 North Island women aged 25 to 39 there were only 90 men, compared with 93 men in the South Island.

But Statistics New Zealand chief demographer Mansoor Khawaja said fertility rates in the South Island were lower than in the north. South Island fertility rates had fallen below replacement with women having only between an average 1.7 to 1.9 babies.

In the north, the figure was just above the replacement rate of two live births per woman. The lowest fertility rate was in Otago, with 1.6 children per woman compared with the highest in Gisborne of 2.5 children per woman.

North Island women also have their first child at a younger age than their southern counterparts, he said.

But Khawaja said in the past three censuses there had been a drift of North Islanders moving to the South Island, for the first time in almost 100 years, lured by cheaper housing and living costs.

The most obvious differences between the two islands was in ethnic make-up. In the North Island 16 per cent of the population identified as Maori, more than double the 7.5 per cent in the South Island, while 8 per cent of North Islanders were Pacific peoples compared with 1.8 per cent in the south.

Only 4.2 per cent of South Islanders were Asian compared with 10 per cent in the north, although there were marginally more Australians in the south than in the north.

Less than one in 10 South Islanders can speak a second language compared with one in five in the North Island, due almost entirely to immigration from non- English speaking countries being greater in the north.

Although overall there was not much between the two islands in educational achievement, 24 per cent of southerners were without a formal qualification compared with 22 per cent of northerners.

North Islanders were also more likely than than their southern compatriots to earn an income over $35,000 a year.

However, both islands were almost identical in terms of marital status, with almost a third of the population never married, 45 per cent who were married and 17 per cent who had been divorced, separated or bereaved.