New Zealand's net migration shrank to an 18-month low last month, suggesting the economic impetus provided by people choosing to live here may be abating.

A net 250 permanent and long-term migrants arrived last month, seasonally adjusted, the lowest since November 2008, according to Statistics New Zealand. Figures for April were revised down to 730 from 770.

Annual immigration slowed to 17,967 in the 12 months ended May 31, the fourth straight month that the annual number has declined, from 19,954 in the 12 months ended April 31.

"Net migration continues to moderate, and arguably at a slightly faster pace than we had expected," said Philip Borkin, economist at Goldman Sachs JBWere. "Visitor arrivals growth is also looking soft relative to our forecasts, with weak arrivals from the UK and Europe, in particular, appearing to be the major drivers."

Departures for Australia rose to 1,693 in May, from 1,241 in the same month last year, for an annual exodus of 15,200 across the Tasman.

Short-term visitors to New Zealand fell 0.4 per cent to 141,300 last month compared to May 2009. Visitors from Australia, the biggest source of tourists, fell 5 per cent, while those from the UK dropped 15 per cent.

Visitors from China, Japan and South Korea rose 45 per cent, which the government statistician said reflected the H1N1 flu pandemic in May 2009, which sapped travel demand.