ABOUT HEAT PUMP HOT WATER SYSTEMS
Heat pump hot water systems work similarly to that of an air conditioner, by taking hot air from its surroundings, and using that to heat water. They operate on electricity, but as they use a refrigerant gas to heat the water and not the electricity directly they are quite a bit more energy efficient than electric hot water systems.
However, for South Australian consumers this may not directly translate to financial savings. Due to their reduced electricity usage & reliance on the temperature, most manufacturers will only connect a heat pump hot water system to your general use electricity tariff instead of the cheaper controlled load tariff that a standard electric hot water system would be connected to. This generally means the savings are minimal, or sometimes you will even be paying more. A lot of information you find online regarding savings are based on QLD or NSW tariffs which are cheaper for day use.
For a detailed explanation on how all types of heat pump hot water systems work see here: https://www.yourhome.gov.au/energy/hot-water-service