Six people were given a low dose of vaccine against Covid-19 at a vaccination site in the Bay of Plenty last month.
No one was harmed, the Bay of Plenty District Health Board said.
The board's senior response officer Brent Gilbert de Rios said a low vaccine dose contained too much dilutant (saline) solution.
Gilbert de Rios would not name the provider that gave the low doses, but said it was "adhering to Tikanga Māori protocol" and working with those affected to address the error.
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"While this process is ongoing, we will respect the privacy of those affected."
In a vaccine roll-out of this scale and dealing with a vaccine that was complex to prepare, such errors would happen, he said.
"Immediate safeguards" were implemented within the provider's processes once this issue was discovered.
The September 20 incident was reported by Covid-19 vaccination and immunisation programme national director Jo Gibbs, Radio New Zealand reported.
The Ministry of Health said those affected in the Bay of Plenty had been contacted and offered vaccination.
The clinical processes and systems around the administration of the vaccination were being reviewed and the health board is working with the provider to make sure it doesn't happen again, the ministry said.