Police say they have arrested and charged a man with arson following the fire at Prime Minister John Key's Auckland electorate office.

The 31-year-old man was also charged with another arson at the Helensville Rugby Club rooms and would appear in the Waitakere District Court tomorrow, said Waitakere police Area Commander Janet Hope.

A small fire broke out when an object was thrown into a room at the Prime Minister's electorate office in Main Rd, Kumeu, about 3am yesterday.

No one was in the building when it was attacked. Two fire appliances attended and those firefighters were able to extinguish the fire before it got out of the room.