A suspicious fire is burning about 10km south of the country's largest Kauri, Tane Mahuta.

But the Fire Service says the giant tree is in no danger from the blaze in the Northland forest.

Firefighters was called to the fire at the Waipoua Forest north of Dargaville about 12.30am, northern fire communications shift manager Dallas Ramsay said.

About 3ha had been burned.

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The blaze began in native bush but had now died down and was "slow burning" in some flax, she said.

The Department of Conservation was now in charge of the scene.