1994 Holden Commodore temp gauge?

I just changed over my water pump(no problems there), took the car for a drive the next day, car ran perfect, gauges read well, the following morning went to town no problem, later that day went to town again, within 5 minutes the temperature gauge suddenly went way past the hot marker an sat over the temp pitchure on the dash, and now as soon as I start the car the gauge returns directly the this position every time. No other lights or warning on the dash, and engine certaily is no overheating? Any idea what might be causing the reading on the gauge like this

thank you