The larvae from an exotic mosquito species has been found near the Kaipara Harbour, north of Auckland.
The Ministry for Primary Industries says it was alerted to the suspected detection of the Culex sitiens larvae last Friday.
Further testing for identification was completed this week.
MPI says further samples are being taken in the area near the initial find to establish if there is a breeding population and, if so, how far it has spread.
If there is a breeding population, the ministry will likely undertake a programme to eradicate the mosquito from New Zealand.