just an idea and maybe a question why could you not run aircon fluid pipes in the ground at --1.5mtrs down and the to air con pump --surely must be more economical as temp of ground in australia for example is 15-17c where it is 30-40c air temp ,run them through some water as well / so must be big eco help if your air con was running at a lower start temp --?

Insulation on the refrigerant lines is much cheaper. I assume that's standard practice in Oz, (just as it is almost everywhere else)? The amount of surface area on those lines is tiny, and losses fairly small even when uninsulated and exposed to 40C air, but insulating them keeps them from dripping condensation, and mitigates the heat gain/loss.

Insulation on the refrigerant lines is much cheaper. I assume that's standard practice in Oz, (just as it is almost everywhere else)? The amount of surface area on those lines is tiny, and losses fairly small even when uninsulated and exposed to 40C air, but insulating them keeps them from dripping condensation, and mitigates the heat gain/loss.

no you missing my point --the refriderant lines on hot side should be buried in ground to lower temp of returning refridgerant before it goes back to heat exchanger ,to remove some heat from the compression cycle.reverse geo heat pump in effect or you could duct the intake air to cooling rad through the ground to lower effective outside air temp. the lines could connect underground to a finned heat exchanger which has some volume to it

Geothermal (ground source heat pump) It's called DX or earth link. Hundreds of feet of copper pipe buried for heat exchange.

I'm not sure how much temp reduction you would get from burying the line set.

thats why I say a larger volume section underground to give more surface area .
even a few degrees reduction would surely make big difference to running costs , just some off the wall thoughts .I understand in most cases the extra cost would be prohibitive over simple wall mounted unit--but technically it must work