The Ministry of Social Development office in Kaikohe has been closed for an unknown duration after an arson attack described by a fire investigator as "senseless, deliberate and malicious".
The attack means clients in need of help in the Kaikohe area will have to contact MSD by phone or online.
Any that need to meet staff face-to-face may have to drive long distances at a time when petrol prices are sky-high and many families are struggling financially.
The Kaikohe Fire Brigade was called out to the government agency's service centre on Memorial Ave, not far from the Far North District Council chambers, just after 2.30am on Monday.
Two crews responded with the first arriving within seven minutes of the call, fire chief Bill Hutchinson said.
Firefighters managed to contain the blaze to a small area but there was significant smoke and heat damage throughout the open-plan office.
It was likely the building would have to close for some time for renovations, he said.
Fire and Emergency investigator Craig Bain said the blaze was "senseless, deliberate and malicious".
It appeared someone had broken a window, ignited an object and thrown it inside.
The Kaikohe brigade did a "brilliant job" by confining the blaze to a small part of the building, Bain said.
Police had taken away significant evidence for laboratory analysis.
Bain said he felt sorry for MSD clients who could now face serious inconvenience.
"They'll have to travel for miles at a time when people are struggling to get from A to B because of the high cost of petrol," he said.
Inquiries by police and Fire and Emergency NZ were ongoing.
MSD regional commissioner Graham MacPherson said, given the level of damage, the Kaikohe service centre would remain closed while the building was assessed.
Staff were making alternative arrangements for those needing assistance from MSD.
"We can assure clients we are there to provide them with a service when they contact us on the phone or online," he said.
Clients were advised to access services in the meantime via MyMSD, by calling 0800 559 009 or by visiting the Work and Income website.