I know a little about oppressive heat, living in North Queensland. I don't have such a big operation as yourself, but we put our hives under shelter so that they get the early morning sun. We either put a sheet of iron up, or put the hive under a tree. We can have decent rain here, so I reckon a bit of shelter also protects them from the often continuous rain. I thought maybe you can prune the lower branches of your trees so you can slip the hives under? Knowing Victoria, though, it's probably gone back to sub-zero temperatures. I did have a sheet of plain foundation melt a little against the side of a hive. Maybe you can try to keep plain foundation off the edges a little.
We also use the taller migratory lids rather than the telescopic lids, mainly for the extra ventilation, and they have ventilation holes in them too.