National's calling for a new Census, saying the information from the last one will mean the 2020 electoral boundaries will be based on guesswork.

Statistics New Zealand is confident the delayed data will be ready for next year's election.

But National's electoral reform spokesman, Nick Smith, told Kate Hawkesby 400,000 people weren't counted in the last Census, the lowest participation since World War II.

"The data from last year is so dodgy [and] the information is so important that we should get on and do a new census in 2021 and make sure we count everybody."

Smith said backfilling the data by using IRD and Work and Income figures will be unreliable, because you don't know who you haven't counted.