New Zealand's population has grown at its fastest rate in 50 years and is expected to reach 5 million next year, according to the first data released from the bungled 2018 Census.

But while a panel of experts says the figures are reliable, it's raised ethical concerns about the way the numbers were put together.

The country's population in 2018 officially reached 4,699,755, jumping by 457,707 since the previous census in 2013 - the largest growth by number since 1961, Stats NZ announced on Monday.

Growth had been at 2.1 per cent a year since 2013, compared to just 0.7 for the previous Census period, with immigration accounting for about two-thirds.

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Stats NZ's general census manager Kathy Connolly told reporters the population was set to hit 5 million at some point next year.

Northland was the fastest-growing region and Waikato the second, while Auckland has now passed 1.5 million people. The fastest-growing districts were Queenstown-Lakes (up a staggering 38.7 per cent to 39,153 since 2013) and Selwyn (up 35.8 per cent).

On the back of the figures, the North Island will get one new electorate for next year's election while about a third of all electorates will be redrawn, the statistic department said.

The Government's statisticians have faced scrutiny since last year's Census – the first to prioritise online data collection - produced one of the worst participation rates in decades, with one in seven people failing to complete it.

That meant Stats NZ had to use alternative data from other Government agencies – such as birth records and drivers' licences - to fill the gaps.

The country's top statistician, Liz MacPherson, resigned over the result in August but has remained to see the clean-up effort.

Following significant delays caused by the problems, the department on Monday released the first set of data it has put together from the Census, MacPherson calling it a "long, hard road" so far.

The department concluded there would be a new electorate somewhere in the North Island for next year's general election. There were no changes to the number of South Island or Māori electorates.

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That will take the number of electoral seats from 71 to 72 (48 to 49 in the North Island), and decrease the number of list seats available in Parliament from 49 to 48.

Where the new seat will go will be determined by the Representation Commission by April next year.

Government Statistician Liz MacPherson presents the first data from the 2018 census in Wellington. Photo / NZME
Government Statistician Liz MacPherson presents the first data from the 2018 census in Wellington. Photo / NZME

Other key figures released on Monday included:

• The North Island's population grew 10.9 per cent from the 2013 census to 3,180,037. The South Island's population increased 9.7 per cent to 1,047,321
• Regions around Auckland have led growth, with Northland growing by 15.2 per cent and Waikato by 13.5 per cent. Auckland itself grew by 11 per cent since 2013.
• Nationally, there was no change to the top five ethnic groups from 2013 to 2018 – NZ European (64.1 per cent) Māori (16.5 per cent) Chinese (4.9 per cent) Indian (4.7 per cent) and Samoan (3.9 per cent)
• The percentage of the population born in New Zealand dropped from 74.8 per cent in 2013 to 72.6 per cent.

How good was the data?

Stats NZ set up an independent panel of experts to assess whether the quality of the data produced in the Census is up to scratch in the wake of the poor response rate.

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In its first report, released alongside the data on Monday, the panel concluded by using alternative Government data, Stats NZ had made a "dramatic recovery" in the reliability of the Census.

It's given the replacement data method a thumbs-up and said, in particular, using figures from other agencies would help groups that were usually under-counted in the Census, including Māori.

However, it's raised ethical concerns about the method - saying the public were not aware data Government departments held about them – such as tax records - would be used.

"The panel excepts the statutory basis of this data use and agrees that it was the only way to deliver any good quality census data," panel member and biostatistics professor Thomas Lumley said.

"However, we are concerned that the New Zealand public was not aware their data was used in this way. There has not yet been any open and comprehensive and open public consultation."

He said there was a fear a lack of "social licence" could impair public trust in future Government data collection.

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And while the panel found the information met the requirement to set electorates, it would fall short on some details and could cause problems in analysis of groups important to the Government - including ethnic communities, local authorities, Māori, and service providers.

It also said data about households and families would be of "low quality" and not comparable to 2013 figures, and information about dwellings would be incomplete because about 8 per cent of the population could not be linked to a particular home.

Stats NZ will not be publishing iwi figures either due to a lack of information, with the response rate among Māori to the Census at 68 per cent, down from 88.5 per cent in 2013.

The remainder of the data will be published in stages by the middle of 2020.

A report earlier this year laid out a litany of problems with how the Census was run, finding too much emphasis had been put on the online component, with a lack of contingencies.