Young people in their early twenties are now the most likely age group to be living in overcrowded conditions, as Auckland's unaffordable housing crisis bites hardest for young adults.

A quarter (25.2 per cent) of all young people aged 20 to 24 in Auckland are now officially considered to be in "overcrowded" housing in the 2013 Census, up from 23.6 per cent in the previous Census in 2006.

The young adult group has surpassed children aged 5 to 9, who were the most overcrowded group in 2006.

Adults are counted as being "overcrowded" if they share a bedroom with someone else other than their partner.

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Children aged 5 to 18 are "overcrowded" if they share a bedroom with someone of the opposite sex. Children under 5 may share a bedroom with either sex.

Statistics NZ senior analyst Dr Rosemary Goodyear said the increased overcrowding among young adults probably reflected increased crowding in both flatting situations and in family homes, with growing numbers of young adults staying at home with parents or in-laws because of the mounting cost of housing.

Young New Zealanders moved out of their parental homes slightly earlier in every census from 1986 to 2001. The median age of leaving fell from age 20 in 1986 to 19 in 2001.

But this trend was reversed in the 2006 census, with a slight rise in the median age of leaving to just over 19.

Dr Goodyear said Statistics NZ had not yet analysed the 2013 data in detail, but pointed to a British report showing that almost half of Britons in the same 20 to 24 age group lived with their parents last year.

A quarter of the entire 20 to 34 age group now live with their parents in Britain, a dramatic increase from one-fifth of the age group before the global financial crisis hit in 2008.

In both countries, the main factor driving young adults to stay at home is believed to be the mounting cost of housing, which has led to a dramatic fall in home ownership.

In New Zealand, 69.5 per cent of young adults owned their own homes by the age of 30 to 34 in 1986.

By 2013, that proportion had plunged to 45.4 per cent, the biggest decline of any age group.