All approved air source heat pump hot water systems are listed on the Governments Clean Energy Registry. To access the registry click here. Once on the Clean Energy Website select Air Source Heat Pumps models link and download the excel or csv file that lists all approved hot water systems;
The downloaded file also lists the number of certificates that can be claimed in each zone. The zones are shown below;
Basically the more certificates the more efficient the model.
Note: The list also contains solar hot water / heat pump combination units which are more efficient than air source heat pump only models, albeit their higher price tag typically makes them less attractive. The ThermalARK products strike an excellent compromise between efficiency and upfront cost.