I am assuming the cistern is over filling into the internal overflow which will cause this problem.
Solution find the fill adjuster and alter to prevent it filling more than 1 inch below over flow.
It may also be the water inlet valve that fills the cistern needs the washer replacing. What type of syphon? cup float type shut off that rises as water level rises to the point of turning off the water, or ball and arm shutt off system.
bob, January 2013
