No dead kiwi have been found at the scene of a fire triggered by a home-made explosive device, the Department of Conservation says.

DoC staff inspected the fire-ravaged flanks of Rangihoua Pā, on Purerua Peninsula in the northern Bay of Islands, on Tuesday in the wake of Saturday night's fire.

Senior ranger Adrian Walker said Rangihoua Heritage Park and the area around Marsden Cross was a stronghold for Northland brown kiwi.

"Some birds could have perished in the fire but there is no direct evidence of this. With little wind at the time hopefully any kiwi present had time to escape," he said.

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Kiwi eggs were particularly vulnerable to fire but no nests were located in the burnt area.

However, the fire had denuded important archaeological features of Rangihoua Pā, making it vulnerable to erosion. Heritage New Zealand and local hapū Ngāti Torehina were assessing the damage.

The area burnt measured roughly half a hectare and had been covered in kikuyu grass, regenerating manuka and kanuka and coastal hardwood trees.

Walker said DoC was disappointed that irresponsible actions had put the reserve's conservation and cultural values at risk.

The fire started about 9pm on Saturday when a home-made incendiary device exploded on the beach. Seven fire appliances and about 30 firefighters battled the blaze on the night; a helicopter with a monsoon bucket extinguished hotspots along the cliff on Sunday morning.

Witness Vanessa Owen said she saw four men, one of whom mooned her, behaving suspiciously on the beach immediately before the explosion. As the fire took hold of the hillside she could hear the "screaming" of kiwi distress calls.

The police investigation is continuing.