Fire and Emergency NZ investigator Jason Goffin said the blaze was suspicious.
One wall had exterior damage and the building's meter box, which was immediately above the fire, was destroyed.
There would be no power to businesses in the building until an electrician could complete the repairs.
There were reports of youths hanging around in the porch before the fire.
It was a reminder for business owners to keep waste bins locked and away from buildings, he said.
Just a few days ago former government minister Shane Jones hit out at "dorks" and an "insidious level of criminality" in Kerikeri after a sculpture commissioned by local hapū Ngāti Rēhia was damaged by vandals even before it was completed.