Typically, an overflowing cistern is most commonly the result of an overflow tube that has become misaligned or at the wrong height in the tank.
As the toilet gets flushed, the flapper is released from the flush valve, and the water in the cistern is released into the bowl. Once the toilet has gone through the complete flush process, the fill valve refills the cistern from your home’s mains water supply. The tank’s overflow tube floating in the middle of the tank stops the tank from overfilling by directing any excess water back to the toilet bowl. If the tube is set too high, or has been misaligned, it might be reset in order to accommodate the surplus water.
Other possible causes are a fill valve that has a leak, a worn or damaged flapper, or a ball float that has become damaged.