The number of net migrants coming into the country since 2001 has been underestimated by nearly 60,000, Statistics New Zealand says.
The Government's statistics department has confirmed a new measure to more closely check patterns of migration has revealed a problem in the older method.
Last year a measure was brought in to track actual travel histories, whereas previous stats were based on what people said they would do on their arrival cards.
While the card-based system showed 300,000 extra people had moved to New Zealand since 2001, the real figure was probably about 59,000 - or nearly 20 percent - higher, Stats NZ has confirmed.
Of that figure, 45,000 occurred in 2001 and 2002.
NZ First leader Winston Peters, whose party has promised to slash immigration, said the Government had explaining to do.
"Minister of Immigration Michael Woodhouse should have been asking questions when New Zealand First kept pointing out the country is being overwhelmed by migrants," he said.
"Statistics NZ have been nothing but amateur sleuths using an outdated method that relies on people sticking to plans, and their honesty."
New Zealand has been experiencing record levels of net migration in recent years, with rising immigration a key election issue as it strains the country's infrastructure and is blamed for inflating property markets.
Net migration rose to a new record for the year to July, according to Statistics New Zealand, reaching 72,400.