The National Party continues to question the figures being used to draw electoral lines for next year's election, despite an expert panel giving the data a thumbs-up.

Stats NZ on Monday released the first of its much-delayed data from the troubled 2018 Census, and announced there would be an extra electorate set up for the 2020 election.

The Census – the first to focus on online data collection – saw the lowest response rate from the public in decades, with one in seven people not completing it.

To fill the gaps, the department used alternative data being held by other Government agencies – such as tax records, birth certificates and drivers' licences - and now says the result is actually better in some ways than during the previous Census, in 2013.

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Its release of population figures this week also included an announcement there would be a new electoral seat somewhere in the North Island in 2020 and that about a third of country's electorates would have to be redrawn to account for moving populations.

Where the new seat will be is due to be determined by the Representation Commission by April.

But National Party has been critical of using the data and its electoral law spokesman, Nick Smith, said the Opposition wanted the Government to reuse the same electorates from the 2017 election.

"It is difficult to have confidence in today's population figures and the decisions on the allocation of General, Maori and List seats when 16 per cent of New Zealanders did not complete Census 2018," Smith said.

"National has doubts as to whether these numbers are sufficiently robust for determining the boundaries for electorates for the 2020 Election."

However, while the party earlier this year said it would be disputing the boundaries, Smith said it would now only be pushing for minimum changes with the Representation Commission, and that pursuing legal options in the case was a "last resort".

"We will be closely scrutinising today's reports," he said.

That comes after a panel of top experts called to assess the quality of the data - in the wake of the problems - on Monday concluded the quality of the statistics for populations was "very high" and approved the method used by Stats NZ.

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The eight-member panel's chair, Professor Richard Bedford, said Smith was simply wrong.

"We have no problem at all accepting the outcomes that were announced today relating to the numbers of electorates ... These are good population numbers and frankly National is grasping at straws," he said.

"It needs to sit down and read the reports really carefully to understand how we came to these conclusions".

Smith said the panel had been appointed by Stats NZ and that the department wanted to minimise the appearance of problems.

But Bedford strongly rejected any suggestion the experts weren't independent or were favouring Stats NZ.

"If you think [this panel] is in the pocket of stats NZ you are absolutely dreaming ... We operated completely independently," he said.

"These are all, in their own areas, very independent thinkers, very independent operators."

Stats NZ has proposed no changes to South Island or Māori electorate ...

The new seat would take the total number of electorates 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. The size of Parliament would remain unchanged.

After the 2013 Census, an extra seat was added in Auckland with the Waitakere electorate being dissolved and new ones created in Kelston and Upper Harbour.