Caroma toilet:water level above overflow tube

The water in the tank of my low-volume flush Caroma toilet is spilling / dripping into the overflow tube. The level of water in the tank is 1/2" (1.2 cm) higher than it should be. This results in a steady trickle of water down the overflow tube, which produces a continuous dripping sound. Any ideas as to how to solve this problem?

The water is shut off by a float valve. These are easily adjusted to stop the water before it gets to the top of the overflow pipe.