The findings had given investigators strong lines of inquiry as they went looking for the culprits, he said.
Meanwhile, Thursday night's rain had helped dampen down the fire but did not extinguish it completely. Firefighters hoped to inspect the area last night using a thermal camera to find any remaining hotspots.
About 15 forestry workers have been on site daily since Wednesday looking for hotspots and putting out flare-ups, including one around noon on Wednesday.
Investigators were also due to head to Taemaro Rd, northeast of Mangonui, to look for the cause of a grass fire put out by the Taupo Bay Rural Fire Party about 5pm on Thursday.
A grass fire at nearby Waimahana Bay in May this year led to the death in hospital some days later of a local resident who had been trying to fight the flames.
As well as being prosecuted for the crime of arson - which carries a maximum penalty of seven years' jail - people found to have deliberately lit a fire can also be billed for the costs of putting it out.
Last summer fires in Northland claimed two lives and cost $2 million to put out.
That does not include the value of destroyed property, such as 350ha of pine trees at Horeke and a number of homes and baches on the Karikari Peninsula.
The cost of sending out a fire appliance starts at about $500; costs rise dramatically once helicopters are required.
A helicopter costs around $2500 an hour to run. In the most serious cases the authority would also take offenders to court.
