The blast followed a twin suicide bombings at food distribution camp on Saturday, in which 30 people were killed.
According to Kohat"s senior police officer Abdullah Khan, the suicide bomber detonated his vehicle outside the police station, destroying several buildings in the area.
"Seven people including a child have been killed and more than 25 others wounded," Khan said, adding that least four policemen were injured.
Local TV report quoted the investigators as saying that 250 kilograms of explosives could have been used in the bomb, which has left a crater of about 2 metres deep on the road.
So far no group has claimed reponsibility for the attack, but the involvement of Taliban militants has not been ruled out.