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.
It says it has successfully eradicated exotic mosquito populations in the past.
Culex sitiens is widespread in South-east Asia, the South Pacific and Australia.
It is known to transmit diseases such as Ross River virus and Kunjin virus, but this is not currently a risk for New Zealand.
MPI officials are investigating how the mosquito may have entered the country.