PIN Timer with power and s/valve for use in Australia $250
In order to fit the solenoid valve (electrically operated water switch), you must have access to the hot water pipe serving the shower and you may need a plumber to cut that pipe and fit the valve.
The 24Volt AC power pack is located near the s/valve so you may need to run an extension cord from the nearest power point.
The keypad (pictured) is easily installed in or near the bathroom. It is a standard power fitting (80 x 120 mm) and will screw to a standard backing plate. No wires are necessary here as it contains a radio transmitter. 5 metres of data cable is provided to hard wire the keypad to the solenoid valve if required but very easy wireless connection is a standard option.
Programme the user PINs and times and they are ready to shower (economically).