A smell coming from your toilet cistern can make using the rest room in your house uncomfortable for both you and your guests. The cistern is the part of the toilet that holds the water prior to a flush. Typically this part of the toilet is referred to as the "tank" in the U.S. and the "cistern" in other parts of the world. Repairing toilet cistern smells is mostly a matter of making sure that the cistern is clean.
Cut the water supply for the toilet cistern with the odour. At the bottom of the cistern, there will be an attached tube or hose. Follow this back to the wall or the floor to locate the water supply shut off valve. Rotate this clockwise to turn off the water supply.
Remove the lid from the cistern. On a dual flush lid, you will have to unscrew the push button flush valve in order to remove the lid. Hold down the handle or press one the buttons on the top of a dual flush toilet to activate the flushing mechanism. Bail out any remaining water and wipe up the remainder with a sponge or towel.
Clean out the inside of the tank. Remove any mould or mildew with a mild bathroom cleaner. Fill the cistern up half way with fresh water and rinse off any bathroom cleaner on the sides of the cistern. Flush the tank and remove the water again.
Fill the tank up with a solution of warm water to at least the top of the siphon or about 5 inches below the tank edge. Pour in 1/4 cup of bleach. Allow this to soak for about an hour.
Flush the toilet to rid the water inside. Restore the water supply by turning the water supply valve counterclockwise.
Install a drop in cleaner into the cistern. There are a number of different ones on the market. These inject a small amount of cleaning fluid into the bowl after a flush and also help to keep the water in the cistern clean.
Clean off the inside of the cistern lid with bathroom cleaner and the rag.
Replace the lid on the cistern.
If you continue to have an odd scent in your bathroom after cleaning out the cistern, the problem could be with a leaking wax seal. Have the seal checked as soon as possible.