Registrar of electors for Dunedin North and South electorates Dee Vickers says the estimate is based on census information and involves those in the 18 to 24-year-old age group.
Nationally, 108,600 have not enrolled as voters.
It was a legal requirement to be enrolled and the Electoral Enrolment Centre has launched a national campaign - Let's Get Loud - aimed at young voters, she said.
Some students might be enrolled in their home electorates, but many were not, she said.
To give some idea of the scale of the gap in the rolls, Ms Vickers said it was close to the number of all registered electors in Maori Hill and St Clair (9300).
It was about one in four young adults.
While low enrolments in the age group was a nationwide problem, with 40% of all those not enrolled in that group, Dunedin looked worse because of its high student population.
Ms Vickers said students often did not realise that if they shifted from one part of the city to another they could be in a different electorate and to qualify for voting in an electorate they had to have been living there for four weeks.
Those who will turn 18 between now and the election can enrol provisionally and would qualify for a vote if they were 18 by midnight before the yet-to-be announced general election day.
Nationally, the centre's campaign to try to get unregistered young voters to sign up will include using social network Bebo and a dedicated Youth Enrolment Week.
Enrolment centre manager Murray Wicks said the campaign would also involve musicians, actors, community groups, organisations and individuals promoting the message it was easy to enrol and voting was also easy, and relevant.
Youth Enrolment Week would run from August 11.
"We want to tap into the networks of young people across the country to get them enrolled," Mr Wicks said.
"This means reaching them online through communities such as Bebo, on the ground where they are studying or taking time out and also in the community and through their families and other networks."
A competition was being run on Bebo to get young people to make their own advertisement about enrolling and voting.
Enrolment forms can be ordered by free mobile phone texting, over the internet or by calling 0800 367656.
People's details can be checked and updated online too, on the elections website.
