Whangarei District Councillor Jayne Golightly has resigned after learning she was not a New Zealand citizen.
The resignation, less than a year into her tenure, will mean a by-election in the Denby ward she represented, but Golightly said she has since applied for NZ citizenship and, if it's through in time, she will contest the by-election.
The council said it will provide more information about the by-election this week.
Golightly said Election Services told her on Friday she was not a citizen.
"It had been brought to their attention that I was not a NZ citizen. To say I was shocked and devastated is an understatement," Golightly said.
"After looking into it, it was confirmed that I was not. Today I resigned from my position as councillor."
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 a NZ citizen.
"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 citizenship. I have never seen myself as anything other than a New Zealander. New Zealand is my home."
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.
"Over the past months I have put my heart and soul into my role as elected representative, and my heels were well and truly planted, getting stuck in on the real issues our district is facing. I am committed to our community. I was honoured and proud to be in my position."
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 be standing in the by-election."
Golightly stood for election as part of the Go Whangarei group.
She is not the first politician to fall foul of citizenship laws, which have led to a number of eligibility scandals in the Australian Parliament.