"Migration flows are volatile from month to month but the modestly improving trend in transtasman migration seems to be firming," Delbruck said.
"This is in line with our view that labour market prospects in Australia are becoming steadily less attractive relative to New Zealand."
A pattern was emerging of fewer departures to Australia every month, said ASB economist Daniel Smith.
"Departures are likely to continue falling as the Canterbury rebuild gathers momentum and the New Zealand labour market improves relative to that of Australia," he said.
Many of those leaving Australia for New Zealand were Kiwis returning home, Smith said.
Overall, New Zealand had a net gain, meaning more arrivals than departures, of 550 migrants last month. That came from 7130 arrivals versus 6580 departures.
Total net immigration had been positive for five of the past six months, Delbruck said.
In the year to February, New Zealand had a net gain of 1200 migrants, up from zero in the year to January, according to Statistics NZ.
"Annual net migration is now firmly back in positive territory and we expect it to rise further from here," Delbruck said. The net loss of migrants to Australia in the February year was 36,700, down from a record of 40,000 in the August 2012 year. That figure came from 52,100 departures to Australia, offset by 15,400 arrivals.
The majority of migrants in both directions were New Zealand citizens, Statistics NZ said.
There were net gains of migrants from most other countries, led by Britain with 5900 people, China with 5400 and India with 4900.
"Inflows of foreign migrants increased late last year but have eased back in the last couple of months," Delbruck said.
"We expect these inflows to become stronger over the next few years as labour shortages associated with the Canterbury rebuild become more acute."
Meanwhile, annual visitor arrivals jumped 8.5 per cent last month to 281,200 - the highest number of visitors ever for a February month.
This increase was driven by a flood of arrivals during the Chinese New Year period. The number of visitors from China rose by 106.4 per cent, from 15,280 in February last year to 31,536 last month.
Arrivals from Hong Kong jumped 144 per cent to 3760.