Cistern leaking water to seat area when I flush

[AndyB97]

Hi Guys,

I have a toilet where when you flush, the water from the cistern leaks out from a section of the cistern at the bottom where the seat area begins at the top just before the bowl.

Was wondering how this could be happening or how it could be fixed?

 

[ShaunCorbs]

Donut washer gone / not sealing anymore