The figures have been calculated by the Herald, based on Statistics NZ data from independent Wellington demographer Dr James Newell, to check election claims by Labour leader Phil Goff that "100,000 New Zealanders have left for Australia" under Mr Key.
Mr Goff repeated that claim yesterday as one of a "top 10 facts" on Mr Key's premiership.
His list also includes higher unemployment and prices, and a $32-a-week widening of the wage gap with Australia over the three years.
"The 10 key statistics I have released today will not improve until New Zealand has the leader who makes the tough calls and does what's right for the future, not what's easy for today," he said.
But Dr Newell's analysis, using rolling totals for the year up to each month, shows a long-term worsening trend in the net exodus to Australia since it last turned negative in December 1991.
Arrivals from Australia have been generally steady at around 10,000 to 15,000 a year throughout that period.
Departures to Australia rose gradually from around the same level to a peak of 42,000 in early 2001, plunged in the panic after the attack on New York's World Trade Centre that September, rose gradually again to another peak of 48,500 at the end of 2008, plunged again in the global financial crisis, and have climbed back steeply to a new peak of 48,800 in the year to September this year.
The departures are rising steadily, dropping to a low of 23,200 after 9/11 but only to 30,500 at the latest low point after the financial crisis, reached in April last year.
Throughout the period, the main reason has been that output and real wages have grown more quickly in Australia than in New Zealand.
"It's about the relationships between the New Zealand and Australian economies, and also about people's perceptions," Dr Newell said.
"It's almost like we talk up this whole thing about 'do you want to go to Australia' because it's every second thing on TV."
He said the outflow was partly just because New Zealanders had become more mobile. The outflow to Australia had been more than balanced by an inflow of migrants from other countries.
His research shows that New Zealand's losses are higher among low-skilled and semi-skilled workers, including tradespeople, than among professional people, who benefit from New Zealand's lower-than-Australia taxes on high incomes.
But almost half of the net loss in the past year was of people in their twenties and thirties, leaving a big hole in New Zealand's working-agedpopulation.
"The impact in terms of accelerated ageing of the NZ resident population and on supply of skilled labour and talent is likely to be a continuing issue," Dr Newell said.
He said only about 3000 of the net loss of 34,150 in the past year was caused by people leaving Canterbury after the earthquakes.
