He said staff did the right thing by evacuating the building and leaving the firefighting to the experts.
The first crew could see the thick and acrid black smoke billowing from the building as they approached, and it would have been a highly toxic mix, fuelled by the burning of the wall linings and insulation, he said.
The flames were extinguished quickly, but two crews from Napier and one from Hastings stayed at the scene for more than an hour to make sure there was no risk of reignition in the building.
HB Seafoods director Nino D'Esposito was in his office in the new block overlooking the inner harbour when the fire alarm activated, and his first response was to call a staff member to turn it off.
"We've been having a few problems with the alarms," he said, "and we haven't had a fire in the whole 20 years we've been here."
He said at least a dozen staff would have been on the premises at the time, and all had evacuated, along with a small number of customers in the shop which had opened about an hour earlier.
While the shop was able to resume trading by early afternoon, the dining area and toilets were out of action. Mr D'Esposito said repairs could be started today.
The two boys spoken to by police later were understood to be about 10-11. Deliberately-set fires in Hawke's Bay have been concerning the fire service and police, who last week charged three teenage boys with arson relating to a fire which destroyed a pirate ship attraction at Hastings theme park Splash Planet recently.
Both fires were during the school holidays.