ACT rentals had the lowest average minimum temperatures at a chilly 7.4C. They were only slightly colder than rental homes in Tasmania, which tracked at an average of 7.5C degrees. The average minimum indoor temperature in Victoria was 9.8C and in NSW it was 10.5C.
That's the more important part. Dropping below 18°C sounds like it's in the 15-18 range. But temps well below 10°C is another story.
I am not sure how often these homes get close to their minimum temperature. But apparently some can't even get their room temperature above 18°C with a heater running.
I'm in Australia in one of the rare high elevation areas (very flat country, only just over 1km up) and I have had ice form on the inside of windows here multiple times a year. I've also had pipes freeze (though luckily not burst) and I've had trouble starting my car in the morning. That said, I've also had a ~28°C overnight minimum which was just awful, absolutely no relief there, so the extremes are fairly extreme here.
Our houses are generally designed with a fairly large overhang of the roof which in winter still lets sunlight in through the windows but in summer shades from about 9am to 4pm, giving you a significant reduction in heat getting in. We also have lots of air flow, much more than in other countries I've been to, so we tend to be a few degrees cooler than outside most of the time. That said, a week of over 30°C every day with night time minimums of at least 15°C, mostly 20°C, will drive you batty.
If 18's the limit, I'm below that most days, and perfectly comfortable. I just wear a jacket if I need to, with a blanket over my legs if it's particularly cold, and I'm nice and cosy.