Residents in three homes near a forest fire in Northland are on standby to evacuate.

The fire, which is being treated as suspicious, was reported at 11.30pm on Monday by residents along Kellys Bay Road in the Kaipara area.

At least 120 people are on the ground today with four helicopters working to contain the blaze.

Steep terrain has made it difficult for staff on the scene to tackle the blaze from the ground and it's estimated to cover an area of around 200 hectares.

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Kellys Bay Road north has been closed to the public, except to residents along that road, three of whom are ready to evacuate if necessary.

The public and residents of Kellys Bay have been warned there is a 40km/h speed limit and lights must be on in this area, due to smoke and dust hazards.