There are a few reasons why your water bill is unusually high. The number one reason for most customers with this problem is a leaking toilet. This is the worst cause for a high water bill! The following are a few things to check for:
You might hear what sounds like your toilet flushing all the time. What is happening is the water in the back tank is not stopping below the overflow. This means that the water level is too high and is constantly filling up. To stop this, bend the rod down. The rod has the black ball attached to it.
In the back tank, there is a black flapper. If the black flapper doesn't sit properly after flushing, this could mean that it is either old or cracked. You must fix this immediately because the water is bypassing the flapper which causes your toilet to waste a lot of water. Unlike the previous problem, you will not be able to hear this happening, so it is wise to check for this problem periodically.
Source: Providince Water Supply