About 3000 people who have been wrongly sent census reminders have been identified via the call centre.

A glitch in the system has meant thousands of New Zealanders are being told to complete their census forms, despite having already done so.

West Auckland's Erin Kingsleigh-Smith said there has obviously been some sort of "cock-up" in the system after she was harassed by Census.

"You assume these things are reliable but obviously they have some teething problems. What I do object to is the threatening notices saying 'you must complete it' when I already have."

Have you had problems with the 2018 Census? Email us at newstips@stuff.co.nz

2018 Census general manager Denise McGregor said about 3000 cases had been identified where people were asked to fill out the census, even though they had already completed it.

READ MORE:

* Census 2018 - filling in the gaps

* What happens if I don't fill in the census?

* What is the census and why should we care?

* LGBTI people will still be invisible on next NZ census

SUPPLIED Denise McGregor says those who believed they completed the Census should say so to the field officer, if they knocked at the door.

"When we are alerted to a case where a household believes they have completed their forms, we can investigate to work out why our system is triggering a follow up."

The reasons for the alerts varied, including households which completed some of the individual forms, but not a dwelling form; cases where letters have been misdelivered to the wrong address; or addresses where there was a record of more than one dwelling at the address.

Kingsleigh-Smith said she completed her family's online census forms almost as soon as she received them.

After that, she tossed out the left-over bits of paper and went on with her life.

"However, a couple of weeks ago I received a pack with a letter saying that my census was overdue. I just ignored it thinking it was a mistake," she said.

"Then a couple of days ago I received another pack with a card saying a Census officer had called at the house."

Kingsleigh-Smith contacted StatsNZ, the ministry in charge of the census, telling them of the error.

"I hope it's all sorted now," she said.

Posting on social media, numerous people have recounted similar experiences of being harassed by Census staff, despite completing their forms.

A Christchurch woman, who declined to be named, said she received three visits by Census workers to her home, four written reminders and three phone calls – despite having filled in her forms.

She was concerned at how much taxpayer money was being wasted on wrongly chasing people up.

"There's some major issues, clearly, right from the get go."

The census was held on March 6 and was an official count of how many people and dwellings there were in New Zealand.

This year Stats NZ encouraged "digital-first" participation in the hopes at least 70 per cent of New Zealanders would log on.

There were delays this year in census codes being sent out to some residents due to cyclone Gita, with 3 per cent of New Zealand households without a paper form or online code on census night.

The Statistics Act 1975 required everyone in New Zealand on census day to take part.

If you didn't participate, or if you provided false or incomplete information, you could be fined.

People who choose not to fill out their census forms can be fined between $50 and $500.

McGregor said if you believed you had completed everything needed for the census and received a visit from the field team, then let the field officer know.

This would be recorded in the system to trigger an investigation and resolution, she said.