Information could also be provided anonymously to Crime Stoppers by calling 0800 555 111.
Fire investigator Craig Bain — who described the attack as "senseless and malicious" — earlier told the Advocate a significant piece of evidence had been found at the scene and sent to a police laboratory for analysis.
Kaikohe 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.
He expected the building would have to close for some time for repairs.
MSD regional commissioner Graham MacPherson said staff were making alternative arrangements for Kaikohe residents in need of assistance.
While the building remained closed, he advised clients to access services via MyMSD, by calling 0800 559 009 or by visiting the Work and Income website.
■ The molotov cocktail was first used in the Spanish Civil War, but perfected by Finnish troops for use against Russian armoured vehicles in the Winter War of 1939. Vyacheslav Molotov, after whom the device is named, was the Soviet Union's second most powerful politician, after Joseph Stalin, during the 1930s.