New Zealand's annual net migration gain has fallen again - continuing a downward trend, albeit at a gentle pace.

"The trend decline in net migration remains firmly in place," said Westpac economist Satish Ranchhod in his analysis of the latest Stats NZ data.

"The annual inflow of people into the country has now fallen from a peak of 72,400 in 2017 to 63,280 now. While that's still high, it's the lowest level since October 2015."

Migrant arrivals were 129,100 and migrant departures were 65,800 for the latest year.

"The number of migrant arrivals in August was only slightly lower than in August 2017, so it was the increase in the number of departures that led to the fall in net migration." Said Stats NZ population insights senior manager Brooke Theyers .

"This is consistent with what we've seen in previous months, with the fall in net migration being driven more by increases in migrant departures than decreases in migrant arrivals."

Meanwhile trend of more kiwi's departing for Australia - which re-emerged in July - continued in August.

Migrant departures for New Zealand citizens were up 600 for the year ended August 2018, to 34,200. Of these, 20,900 were to Australia.

This increase in departures was accompanied by a fall in the number of citizens returning to New Zealand from Australia, which led to a net outflow of 1,200 migrants.

"New Zealand's net migration with Australia is typically negative so dropping below zero is not unusual." Theyers said.

"Historically, New Zealand citizen departures have been much higher. For example, in 2012 there were 48,700 New Zealand citizens who departed to Australia, which led to a net migrant loss of 40,000."

Westpac's Ranchhod noted another factor weighing on migration "is departures of non-New Zealand citizens to areas other than Australia, which are continuing to trend higher."

Longer term he said he expected net migration would continue to gently ease over the next few years.