A byelection will cost about $35,000 in Whangarei's Denby ward after councillor Jayne Golightly's shock resignation.
The councillor has offered to pay for the byelection - but would not be allowed to.
And, in another development, Golightly said she would try to fast-track citizenship so she could stand again for election.
The resignation, less than a year into her tenure, means a byelection will be held in the Denby ward on December 7, at a cost of about $35,000, electoral officer Dale Ofsoske said.
Ofsoske said a councillor resigning after election because they were not a New Zealand citizen was very rare.
"I'm not saying it doesn't happen, but I'm not aware of it happening in my 30 years [of working for Election Services]," he said.
Golightly said Election Services called her last week, seeking confirmation of her citizenship.
"To say I was shocked and devastated is an understatement,'' she said.
"After looking into it, it was confirmed that I was not an NZ citizen."
She said she emigrated from the United Kingdom with her parents when she was just 18 months old and although she travels on a UK passport, she believed she was an New Zealand citizen.
She said she had since applied for citizenship and is hoping to get it fast-tracked so she can stand in the byelection.
Golightly said she had offered to pay the costs of the byelection but Ofsoske said a byelection had to be funded by the relevant council, not a potential candidate.
Ofsoske said concerns were raised about Golightly's citizenship to him and he immediately contacted her to clear the matter up.
Nominations for the byelection open on September 13 and close at noon on October 11.
Golightly will have to have her citizenship ceremony before nominations close to stand.
''My mother and father were granted citizenship in 2003, when I was on my OE in the UK.
"I had absolutely no reason to ever question or suspect my lack of NZ citizenship.
"I have never seen myself as anything other than a New Zealander. New Zealand and Whangarei have always been my home,'' Golightly said.
She said the mistake was entirely her fault.
''I want to apologise wholeheartedly to our district, to voters, and the people of Denby who choose me to represent them as a WDC councillor,'' Golightly said.
She spent Tuesday in Auckland with the Department of Internal Affairs and filled out paperwork and documents applying for citizenship.
"And I was granted the grounds for an urgent application to my immense relief.
"My application is being processed and I will travel nationwide if I have to for my citizenship ceremony. I will absolutely be standing in the byelection.
"I'm not trying to brush this off and it was a mistake on my behalf, but I'm not giving up.
"People who know me know it won't make me give up and I've really loved my time on the council. It's something I've put my heart and soul into and I want to make things up to those people who put their faith in me and voted me in."
Golightly stood for election as part of the Go Whangarei team. In September last year, before October's local body elections, Go Whangarei candidate Angela Gill withdrew her candidacy after she was mistakenly advised she was eligible to stand for council, as she was part-way through obtaining New Zealand citizenship.