The second month of the new net migration statistics suggests the data will be highly volatile and that at the very least, a longer build-up of history will be needed before anybody can rely on each new month's figures.
The latest release shows Statistics New Zealand's provisional estimate is that annual net migration in calendar 2018 was 48,300 people, down from a still provisional estimate of 52,700 in 2017.
Migrant arrivals are provisionally estimated at 145,800 in 2018 while migrant departures are estimated at 97,500.
The government statistician calculates the true net figure for 2018 could be plus-or-minus 1,800 people and it revised the year-ended-November figures to 48,000, which it says remains an estimate.
That's compared to the 43,400 figure for the year ended November it reported last month, so the revision was a much bigger increase than last month's plus-or-minus 1,500 caveat.
Stats NZ says its new November estimate still has a plus-or-minus 1,200 people caveat and each month's numbers won't be finalised until after 16 months.
In 2018, most migrant arrivals were from Asia or Oceania and New Zealand citizens were the largest group, accounting for 36,500 arrivals, followed by China with 15,700 and India with 14,400.
New Zealanders also accounted for the largest group of migrant departures at 44,700, followed by China with 10,500 and Britain with 5,900.
The government no longer requires people leaving or arriving in New Zealand to complete the arrival or departure cards from which the data was previously compiled.
Because people's intentions often change, their own estimates of how long they intend to remain in New Zealand probably over-stated the extent of net migration.
"The revisions show the challenge of estimating migration independent of passenger cards," says population insights manager Brooke Theyers in the statement announcing the new data.
"With nearly 14 million border crossings in 2018, small differences in classifying travellers to short-term or long-term can affect the provisional estimates," Theyers says.
Of every 50 people crossing the border, typically 49 are short-term visitors and only one is a migrant arriving or departing. Of the 14 million border crossings in calendar 2018, only 81 per cent are currently classified with certainty, she says.
In the December month alone, only one in four arrivals are classified with certainty but that will increase to nine out of 10 after four months.
"Therefore, we expect the monthly revisions to become relatively small after about five months as we can calculate the duration of stay/absence more definitively."
The model will evolve as the government statistician gets more data. "A key next step is to take the new series further back in time to quantify the variability in the provisional estimates," Theyers says.
Stats NZ has taken the new series back to May 2015 and has final estimates through to August 2017.
It also has an experimental outcomes-based series going back to December 2001 through to June 2017 to provide a longer time series.
Stats NZ confirms that annual net migration peaked at 63,900 in the year ended July 2016, a year earlier than the previous passenger card-based data.
The new data for the five years ended December 2018 shows a record net migration gain estimated at a net 270,000.
That comprise an estimated 700,000 migrants arriving and 430,000 departing over the five years.
Over those five years, most migrants arrived on work, visitor or student visas and stayed for at least 12 months after extending their visa or transitioning to other visa types including residence visas.
"Even though many migrants arriving only stay for a year or two, it's important to count them as migrants and not short-term visitors. They are part of our resident population, which has implications for infrastructure and service provision," Theyers says.
The net loss of New Zealand citizens in 2018 is estimated at 8,200 although Stats NZ says that could change by plus-or-minus 900. That's well below the annual average net loss since 2001 of about 18,000 New Zealand citizens and well down from the record 36,200 net loss in calendar 2011.