The Electoral Enrolment Centre says about 241,000 people are not enrolled for October's elections for councils and district health boards.
The centre also says tens of thousands more will miss out because they have moved house and have not updated their address.
National manager Murray Wicks says enrolment is lower than average in Palmerston North, Hamilton, Wellington, Christchurch, Dunedin and the Auckland super-city area.
All those enrolled and eligible to vote in council and health board elections, will receive an update package in the post this week.
Auckland has the greatest number of eligible electors who are not yet enrolled to vote.
The Electoral Enrolment Centre says almost 100,000 people in Auckland are yet to enrol.
The enrolment rate is 91% - one of the worst in the country.
Local government centre director at the Auckland University of Technology, Peter McKinlay, says voters may not realise how much power the new council will have.
He says the new mayor will probably be the second most influential politician in the country, behind only the Prime Minister.