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.
The worst thing I see across the country is the lack of fly screens with security screens on them. I grew up in a house with them (Southern Queensland) and the were a fantastic and free way to moderate the temperature, you could leave them open all day when it's warm or just during the day during winter and not have to worry about people breaking in.
Anecdotal, but I don't think I know a single person here who is hesitant to use heating/AC - could just be down to the people I know, but I get funny looks when I tell people just to wrap up.
Something that did surprise me here is the near-complete lack of double glazing. Everywhere I've lived has been thin single-glazed windows which just let out all of the heat/cold. It's bizarre.