Police had been called in police because there was no natural source of ignition, Mr Kerrisk said.
A newspaper delivery driver is thought to have been first to raise the alarm, followed soon afterwards by security guard Anthony Kenny, who saw a column of smoke rising above the trees as he was driving down School Rd.
"As I got closer I could see the flames going up the side of the building. It was well and truly alight."
Far North Mayor John Carter said the fire could have destroyed the building if it had another five minutes to take hold.
He thanked the Paihia, Kawakawa and Kerikeri Volunteer Fire Brigades, saying he was impressed that the Paihia brigade had 10 firefighters at the scene within 10 minutes of the 4.40am emergency call. They then put the fire out within 10 minutes.
"I want to extend our heart-felt gratitude to our volunteer firefighters who provide an amazing service to the community without payment," Mr Carter said.
He was also grateful to Mr Kenny and the courier driver who reported the fire and tried to put it out.
"We've been told that we could have lost this building if the fire had burned another five minutes and spread inside," he said.
The fire did not damage the library itself but books, fittings and walls were affected by smoke. Staff spent Thursday and Friday removing books so professional cleaners could get to work during the weekend. It was hoped the library, excluding the children's area, would re-open this week. Repairing the structural damage would take at least a month. Council staff were working with Heritage NZ and Friends of Williams House to make sure the historic building was not altered.
Williams House was built in 1920 by Percy Williams, grandson of the pioneering missionary Henry Williams (1792-1867). The house and Village Green was sold to the then Northland Harbour Board in 1967 and handed over in 1993 after the death of its last occupant, Mary Williams. It is owned by the Far North District Council.
Anyone with information about the fire, or who saw suspicious activity near Williams House about 4am last Thursday, is urged to call Detective Sergeant Sam Bindon at the Kerikeri police station on (09) 407 9211.