Residents in the north Waikato town of Tuakau are being evacuated and others are being advised to stay indoors after a large amount of an unknown chemical entered drains near the water treatment plant.

Fire and Emergency (FENZ) said fire-fighters had responded to the "hazardous substances incident", which was reported at 4pm.

In a statement, FENZ said residents have been evacuated from houses in the vicinity of Kowhai St and Matipo Drive.

"Other people in the area are advised to stay inside with doors and windows closed to avoid possible exposure to fumes. Other members of the public should stay away from the area until further notice."

Police have assisted FENZ with the evacuations.

Specialist advice has been sought from the Queensland Fire Department to help identify the substance and FENZ was working with the Waikato District Council and Watercare to develop a plan to flush the drains safely.

"The nature and origin of the substance is not yet known," said FENZ.