"Doctors have confirmed the death of seven people, while an eighth is in hospital in a serious condition," Magdalena Bilinska, a fire brigade spokeswoman in the village of Karczowka, told Poland's TVP public broadcaster.
The accident occurred on a family farm as the victims were reportedly trying to empty the tank, according to TVP.
A woman reportedly drove a tractor onto the tank's cover, which gave way.
Seven family members who tried to rescue her were likely overcome by noxious gases and also fell into the tank, which measures 50 square metres by three metres deep.