It sounds like the ball **** valve may be borked.
It may be that the float at the end is not floating properly, or as above, the value just lets in water and the tank is overfilling.
Need to source out the tank and get the lid off if you can.
Run the taps down enough to stop the overflow outside, then leave them running for another 5 mins.
Return to the tank and manually lift up the ball ****, if the water stops filling then bingo....
HIH.