The big thing here seems to be that it uses CO2 as a refrigerant. I had a Hitachi Yutaki air-to-water heat pump installed earlier this year, the model generation was first released in 2015, so it's kind of old tech now. It has a SOP of up to 5.25 depending on the configuration, and works down to -25c in heat pump mode.

I went with the R410A version, as the R32 version has hydraulic lines between the indoor and outdoor units, and I didn't want to risk them freezing during winter (where I live it's a little colder than Australia). I guess CO2 heat pumps are the same, and they just haven't got around to releasing the reversible version yet.

(I wouldn't recommend this model however, the software is pretty bad. It can't even calculate what SOP it is operating at. I wanted to get Panasonic, but my contractor insisted on a company that supplies Hitachi instead).