The net inflow of migrants has dwindled and finally turned negative over the past year, but Westpac economists expect it to pick up strongly next year as the Australian labour market cools and New Zealand's picks up.
Of those two drivers, the state of the Australian jobs market is much the more powerful, their research indicates.
They have constructed a model which fits the pattern of the past 10 years closely. It shows that a 1 percentage point fall in the Australian unemployment rate directly boosts departures from New Zealand by 2500 in the quarter, while a 1 percentage point rise in the New Zealand unemployment rate only boosts departures by about 800.
In the year ended October there was a net outflow of 35,000 New Zealanders to Australia. Over the past 10 years it has averaged 22,000.
Within those net figures it is departures from New Zealand, rather than returning Kiwis, that is the more volatile influence, ranging from 21,000 in 2003 to 44,000 in the latest year. Australia's unemployment rate, which was below 5 per cent at the start of the year, has risen to 5.2 per cent and Westpac economists expect it to hit 5.6 per cent by the middle of next year.