The main idea is that the target should not have any shadows cast on it, even the slightest bit of shadowing so that it is evenly lit across it's surface. A cloudless day ensures that the intensity of the light on the target doesn't change as it would if a cloud moved in front of the sun at the moment before you release the shutter. A consistent overcast wintery day when the clouds are heavy works just as well imo, as it provides even diffuse lighting on the target, as long as the overcast is heavy so to speak and not a thin overcast sky, where any movement of the clouds may more easily change the intensity of lighting on the target. I suspect in Australia, you don't have to wait long for a sunny cloudless day.