Surprise Census figures suggest that poverty may be breaking up the nuclear family.

An ethnic breakdown of Census figures, bought by the Herald, has uncovered stark differences in partnering rates for women in the prime parenting 25-44 age group, which appear to be related to levels of education and income.

Three-quarters of European and Asian women in this age group said on their Census forms last year that they lived with their husband, wife, civil union or de facto partner, boyfriend or girlfriend - 75 per cent of Asian women and 71 per cent of European women. But less than 60 per cent of Pacific women lived with a partner, and for Maori women it was only just over half - 53 per cent.

More than 60 per cent of Maori women in four Auckland local board areas north of the harbour bridge lived with partners, but the proportion dropped below 45 per cent in the five poorest local board areas in South Auckland.

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The income effect was also clear nationally, but it was muffled by relatively high partnering rates in rural areas compared with cities. Seven of the lowest partnering rates for Maori women were in Auckland local boards and the other three were in Hamilton, Wanganui and Kawerau.

Wellington analyst Paul Callister and Statistics NZ demographer Robert Didham said in Auckland poverty was increasingly concentrated because of housing costs.

"What you are seeing in Auckland is a real sorting effect in the housing market, it's pushing the sole parents into certain areas," Dr Callister said.

He said the welfare system meant many couples were better off by separating. Welfare entitlements are based on family income, so if one person loses a job they can't get a benefit if their partner is working.

Former Children's Commissioner Dr Cindy Kiro, who co-authored a report on Maori families in 2010, said the low partnering rates for urban Maori women reflected a weakening of extended family ties compared with rural Maori and urban Pacific women.

Education is also a powerful factor. Partnering rates for men aged 30 to 44 plunged by 23 per cent between 1986 and 2006 for men with no qualifications, but by only 7 per cent for those with tertiary qualifications.

Another factor is that Maori have emigrated to Australia at a faster rate than other New Zealanders. Maori men aged 25 to 44 in New Zealand declined by 5.6 per cent between the 2006 and 2013 censuses, twice the 2.8 per cent decline of Maori women.

Maori men aged 25 to 44 have dropped from 85 for every 100 Maori women of that age group in 2001 to just 81 for every 100 women today. Men aged 25-44 are in short supply in all four main ethnic groups, but the ratios are higher for Asians (85:100), and Europeans (89:100).