Sewage backing up indoors means the drain outside is blocked and the property is now the lowest available overflow point. Until the blockage is cleared, anything that goes down a drain in the building has nowhere to go but back up.
Wear gloves and eye protection if you must be in the room, open windows, and keep the door closed to the rest of the house. Do not attempt to plunge a WC that is already overflowing.
After the drain is cleared, the affected area needs proper disinfection. Photograph everything before clean-up if you intend to claim on insurance.