General Mohammad Hussein Andiwal, Helmand province police commander, told the AFP news agency that the attackers had somehow passed through security to plant the bomb.
"How it happened, we don't know but some explosives were planted inside the building which caused the explosion," he said.
"As a result of the explosion, two employees of the department have been killed, six other employees and two civilians have been wounded."
The one-storey building, made from traditional mudbrick, collapsed after the blast, which caused scores of people to gather at the site, according to witnesses.
The building, about 200 metres from a small hydropower plant, was where people came to pay their electricity bills or contact officials about their power supply.
The power supply was not interrupted by the attack.