At the same time as fewer Kiwis were leaving to go to Australia, more migrants were heading back the other way, said ASB economist Daniel Smith.
"We expect inflows will continue as the NZ labour market improves and Australia's struggles to add jobs at a sufficient pace to keep up with population growth," Smith said.
"NZ's labour market is likely to remain relatively healthy compared to those in Australia and Europe, which will continue to drive inwards migration."
Overall, New Zealand had a net gain of 15,200 migrants over the 12 months, compared with a net loss of 3,300 in the previous year.
The highest number of migrants by country came from the United Kingdom (6,000), ahead of China (5,400), and India (5,100).
Westpac economist Felix Delbruck said the net inflow was even stronger than expected.
"If recent trends continue, annual net immigration will easily surpass 20,000 by the end of this year," he said.
"And, with unemployment in Australia expected to hit around 6.5 per cent next year, we expect net immigration to rise even further in 2014, which would make this New Zealand's biggest migration cycle since the early 2000s."
New Zealand's unemployment rate rose to 6.40 per cent in the second quarter, from 6.20 per cent in the first quarter.
Delbruck said the inbound numbers were "big enough to matter for the housing market over the short to medium term".
More people arriving than leaving would put pressure on house prices, despite new lending restrictions and recent rises in fixed-term mortgage rates, he said.
Meanwhile, visitor arrivals to New Zealand were the second-highest ever for a September month, pipped only by September 2011 when the Rugby World Cup was in full swing.
A total of 191,100 visitors landed here last month, compared to 179,100 a year ago.
Chinese visitors boosted the overall figure, said statistics manager Andrea Blackburn.
"The 21,200 visitors from China was well up from 14,000 last September," she said.
"This continues the strong growth in visitor numbers which we have seen from the world's most populous country in recent years."
Visitor arrivals in the year to September were 2.670 million, up 3 per cent on the previous year.
United States visitor numbers were up by 11,400, compared to last year. There were also more visitors from China (up 49,600) and Australia (up 23,300).
New Zealanders took 211,400 overseas trips last month, which was 2 per cent higher than a year ago and the highest ever for a September month.
