Key Points:

A fire that extensively damaged a Northland pub is being treated as arson, investigators said today.

Around a dozen appliances were called to the property at Okaihau, 34 kilometres north-west of Kawakawa, early yesterday morning, after a neighbour alerted emergency services at 4.50am.

Senior fire safety officer Craig Bain said the signs were that the fire was started deliberately.

He said Kaikohe police would want to hear from anyone who happened to be driving along State Highway 1 past the pub between 4.30am and 5am yesterday.

"Kaikohe CIB would be interested to hear from any motorists who went past, whether truckies, people away on holiday, or whoever," he said.

"What they saw will help us to build a picture and a timeline."

Mr Bain declined to say how the fire was likely to have been started, but said witnesses reported that it caught hold quickly.

"All the people we've spoken to all reported that there was a very big fire very, very fast," he said.

"A normal smouldering fire wouldn't work like that."

He said the bar area was damaged to the extent that it would probably end up being demolished.

However, a firewall had done its job in preventing the flames from going through to the accommodation section, which was used by the owner and his family.

There were "three or four" people in the accommodation area at the time and they escaped physical injury, but were in shock.

Mr Bain understood that the pub wasn't insured.