Has anyone heard of earth tubes?
Basically they are pipes you place about 3 metres below the ground in open ground, or better still, only a few inches below the house. These feed air into the house.
At these depths, the soil temperature, in Sydney, is 17 Celsius all year round. There's not a lot of cooling reserves, but combined with high thermal mass building, sun shading and night ventilation should provide that little bit of cooling to keep the house temperature comfortable even on the hottest summer's day. (Also handy on a winter’s night if the house temperature ever drops below 17 Celsius.
Sound like a great idea and if put in before construction don’t look like they’ll add too much to the building cost.