Population growth from migration is continuing to decline with the net gain, long term arrivals minus long term departures, dropping to its lowest level in four years.

Statistics NZ said there was a net gain of 6263 people in September, which was the lowest it has been for the month of September since 2014.

That's down 21% from the September 2016 peak of 7904.

On an annual basis there was a net gain of 62,733 in the 12 months to September, the lowest it has been for that period since 2015, and down 12% from the September year peak of 70,986 in the 12 months to September last year.

The fact that the monthly net gain figures are falling at a faster rate than the annual figures suggests the numbers are continuing to decline.

The reduction has been caused by a modest drop in the number of people arriving in New Zealand on a long term basis, which was 128,982 in the year to September, down 2% compared to the previous 12 months.

The biggest decline in new arrivals was from China, with long term arrivals from that country falling to 10,978 in the year to September, down 9.5% compared to the previous 12 months.

Over the same period there was a big increase in the number of people leaving this country long term, which was up 9.3% in the year to September compared to the previous 12 months, with China also recording the biggest change in departures.

In the year to September 1008 people left NZ long term for China, up 41% compared to the previous 12 months.

However China remains the biggest single source of new migrants, with a net gain 7517 from that country in the 12 months to September, followed by India 7231, South Africa 5304, the UK 4955 and The Philippines 4348.

New Zealand citizens continue to leave the country in greater numbers than those who return after an extended stay overseas. There was a net loss of 2767 New Zealand citizens in the year to September, and a net gain of 65,500 non-New Zealand citizens over the same period.

The biggest group arriving by visa type were those on work visas, and their numbers are continuing to increase.

In the year to September 46,933 long term arrivals were on work visas, up 2.5% compared to the previous 12 months.

But people coming on residency and student visas are declining.

In the year to September there were 23,415 arrivals on student visas, down 2.6% on the previous 12 months, and 13,169 arrivals on residency visas, down 18.6% on the previous 12 months.

In a newsletter on the migration figures, Westpac senior economist Satish Ranchhod said migration was expected to continue falling.

"We expect that migration will continue to ease back over the next few years, pulling population growth down in the process," he said.

"This reinforces our expectations for a period of soft demand growth over the coming years."