Let's see if I've got this right. You are hooked up to city water and your water tank has filled itself and is now overflowing. Is this correct?
If this is what is happening the most like cause is the check valve in the water pump. Lot's of things can cause this to happen - incoming water pressure is high and is forcing water past the valve, a bit of dirt has caused the valve to stick open, or maybe the valve has just worn out. Fixes are relatively simple - add a check valve between the tank and the pump inlet, rebuild the pump, or replace the pump.