Your solar hot water system has a temperature and pressure relief valve near the top of the tank as a safety feature.

During periods of high temperature and low hot water use the solar HWS can reach very high temperatures.

Sometimes the pressure relief valve will open and discharge excessive hot water. This is the safety valve doing what it should do.

You can turn off the water to the solar HWS, or at the mains and wait a few hours for the system to cool down.

Once cooled you can turn the water supply back on and everything should be fine.

Otherwise the valve should close by itself once the hot water has discharged. It can discharge as much as 200 litres before closing.