I lived in Canberra, which sits in the middle of a valley so it got very cold in winter (for an Australian city) and uncomfortably hot in summer. Wikipedia tells me that lowest average low is −0.1 C in July and highest average high is 28 C in Jan. In winter heating was a necessity. And in summer owing to lack of any source of breeze AC was 'nice to have'.
I now live on the South Coast of NSW much more temperate climate 8 C to 26 C I use neither AC nor heating we have a nice coastal breeze most days during summer so is not actually that unpleasant. This is quite similar climate-wise to Sydney though you would not get cooling effect of coastal breeze if you lived there so A/c probably more desirable but I would say not essential.