In the north and in the Center it was brutally hot, over 90 F. regularly. With crushing humidity in the North. Sydney was pleasant, though creeping up into the 80's by the time I left. The southern coast could get chilly, breezy and in the 50's, or be sunny and in the 70's. Temperatures in Australia of course are reported in Celsius, which translates to low 20's being nice, high 20's getting uncomfortable, 30's being yucky.
Spring is probably the prettiest time to be there.
Everywhere in the South was lush and green. By summer the hills would be burnt and brown. If you can get to Western Australia in the first half of September, do it. The wildflowers are amazing. I saw the barest hint of what was left, at the end of the month.