They forced the door to get into the Eagle Star offices in Dockray Street and, after a search, stole the keys to two vehicles parked outside and drove them off.
One of the cars was found abandoned in Nelson the next day, but the other, a Toyota with the registration number W173 LMS, is still missing.
The incident occurred bewteen 3-10 and 3-55 a.m. and police are appealing for information.