New Zealand's net migration rate for the year to April 2019 rose to 55,800 from 50,200 in April 2018 - up more than 10 per cent.
A new statistical model has meant some ongoing revision to the figures, but the latest data indicated a "second wind" for net migration was still in play, said ASB senior economist Mike Jones.
If it continued it could have implications for the Reserve Bank's assumptions about the economy which are based on falling net migration, Jones said.
Annual net migration has remained at high levels since the December 2014 year, StatsNZ said today.
"Since late 2014, annual net migration has ranged between 48,000 and 64,000," population indicators manager Tehseen Islam said.
"The only previous time net migration was at these levels was for a short period in the early 2000s."
There has been some ambiguity about net migration data since Stats NZ switched to a new "outcomes" based methodology - based on electronically recorded arrivals and departures rather than the stated intention of migrants.
But Stats NZ chief demographer Kim Dunstan said the model being used to make estimates was now into territory where it could be considered robust.
"Annual arrival and departure data are subject to notably less revision at 5–6 months after the reference period," he said.
"By this time there is more certainty about whether travellers are short-term or long-term (migrants), according to the 12/16-month rule."
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The outcomes-based measure was introduced in September last year.
Where the peak in net migration numbers had previously been recorded at 72,000 in mid-2017 (right at the height of the election campaign) the new data says the record was just 64,000 set in mid-2016.
Dunstan described the change as being from "having very precise but not necessarily the most accurate numbers to having something that has uncertainty, at least initially, but which is a more accurate measure because it's based on actual outcomes".
Given the time it takes to be sure someone really has stayed long term, it is generally considered that it takes 12-16 months before you can be 100 per cent sure of the data.
But in order to maintain a timely flow of data, Stats NZ is modelling the results which are expected to get more robust as the months pass.
"By five months the overwhelming majority of arrivals can be sorted into long and short term and the numbers become stable."
That uncertainty saw StatsNZ today revise its provisional estimate of net migration for the November 2018 year is 51,200.
"Interpreting monthly net immigration data remains difficult," ASB's Jones said.
"Monthly net immigration inflows ticked up in April, but only because prior months were again revised lower. This nevertheless continues the reported net immigration 'second wind' in play since mid-2018."
"The longer this reported upswing goes on the bigger risk it is to our and the RBNZ's expectations for net immigration to trend lower over the next few years," Jones said.
Jones said he expected the RBNZ wil continue to look through the upswing for now but he noted that it was something with the slowdown in housing and consumer spending had been attributed.
With GDP growth slowing an ongoing high level of net migration means we could effectively be experiencing no growth on a per capital basis.
That would be a concern for the Government.
In opposition both Labour and New Zealand First campaigned on lowering net migration numbers.
Since taking power the Government has not moved away from large structural rule changes to restrict immigration.
It now says it is focused on the quality of immigrants and efforts to align policy with areas of skill shortage.
For the year ended April 2019 (compared with year ended April 2018) provisional estimates were:
Migrant arrivals, 151,000, up 7 per cent
Migrant departures, 95,100 up 4 per cent
Annual net migration gain, 55,800, up from 50,200.
For migrant arrivals in the April 2019 year, New Zealand citizens were the largest group with 35,900 arrivals. The next largest groups were citizens of:
South Africa: 8200
United Kingdom: 7500
For migrant departures in the April 2019 year, New Zealand citizens were the largest group with 46,100 departures. The next largest groups were citizens of:
United Kingdom: 6000