Caroma cistern under water valve.
There are many different types of under water valves, depending on the make.
This cistern is a Caroma. The problem was that the cistern continually flushed. On checking inside the cistern lid, the plastic retaining clip was preventing the valve from dropping down and sealing the outlet of the cistern. This was probably due to the push button being pressed so hard that it caused the retaining clip out of alignment. After removing the valve and realigning the clip it was good as new.
Another happy customer. Meeting your plumbing needs.