Low RH and high temps are, like MortonH said, less uncomfortable. But still, 45 is damn hot no matter what the humidity is. I live on the eastern side of the Mount Lofty Ranges and the humidity varies quite a bit.
On Thursday at home it got to max 44.7 and humidity of 7% during the day. At about 22:30 it was still 28 and humidity of 13% which made for a dewpoint of -3. Plus there was a slight (6 - 9 kph ESE) breeze. Was a good night outside.
I use this calculator
http://www.dpcalc.org/.