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Almost a quarter of New Zealand's most highly skilled people have left the country - the biggest exodus of skilled workers from any developed nation.

A new report by the Organisation for Economic Cooperation and Development (OECD) takes official statistics from its 30 member nations to produce data on the numbers of expatriates from all countries.

It shows that 464,465 people born in New Zealand were living in 26 other OECD countries at the last censuses in 2000 and 2001.

The figures also show that 24.2 per cent of all New Zealand-born people with tertiary educations now live overseas.

Only two other nations come anywhere near this level of "brain drain" - Ireland (also 24.2 per cent, though marginally behind New Zealand) and tiny Luxembourg (22.2 per cent).

New Zealand's brain drain is nearly 10 times worse than Australia's. Only 2.5 per cent of tertiary-educated people born in Australia live outside their homeland.

The actual number of Kiwi expatriates is even higher than the figures indicate, because they count only OECD countries. They also exclude three OECD nations which did not count the numbers of New Zealand-born people separately - Germany, Italy and South Korea.

The report comes days after Immigration Minister Paul Swain signalled a campaign using family members to tell skilled young expatriates about jobs at home.

No age breakdown of the expatriates is available, but Waikato University population expert Richard Bedford said they would include both young people on short-term overseas experience (OE) and others lured by bigger overseas incomes to help pay off student loans now averaging $14,424 for each student.

"Certainly that is the case for some people prepared to live in relatively harsh conditions in the United Kingdom where they can earn big salaries," Dr Bedford said.

However, New Zealand's figures also reflect the country's small size alongside a bigger neighbour. Three-quarters (77 per cent) of all NZ-born expatriates have just hopped across the Tasman to Australia.

This means there are more New Zealanders outside the country (464,465) than Australians outside Australia (328,405). There are 355,765 Kiwis in Australia but only 56,142 Australians here.

Both countries have a lot more foreign-born immigrants than emigrants. Even if New Zealanders are excluded, Australia has 3.7 million foreign-born residents (21 per cent of its population), while New Zealand has 624,405 foreign-born (18 per cent), excluding Australians.

The report shows that immigrants to both countries are more highly educated than the average Australian or New Zealander, reflecting selection systems in both countries which target skilled migrants.

But expatriates who leave both countries are even more highly educated than the immigrants, and better qualified than expats from anywhere else, except the US and Japan.