Holy smokes... 30 Australian cents per kw? That's robbery

! Comes out to roughly $0.22 US. Here in Houston electricity is deregulated, meaning one parent company provides the infrastructure & others resell it. This creates a competitive market where you can pick your energy "provider." Mine runs about 10 to 11 cents/kw on average (less if there's a promo for switching providers), so 13-14 cents Australia.
What we deal with here in Houston is humidity. We're called the Bayou City for a reason. It's a freaking swamp. It gets 100+ degrees Fahrenheit, and the humidity can reach 100% on the scale.
Even if it is 77 F outside, it might be 80% humidity so we have to turn on AC units on 74 or so, to cut on the machine and "suck" out the humidity. We typically have central air ran throughout the house versus dedicated room/window units.
Also, if an AC is oversized here, it will get the room down to set temperature too quick, not allowing the humidity to be removed, creating a damp and cold room. This is what happens in a lot people's sheds/garages when they decide to put a big unit in it.
On the flip side, too small of a unit will be overworked.
For comparison, my house is ~2,400 Sq ft and I have a 3.5 ton, 42,000 BTU, 16 SEER centralized unit. My garage isn't included in that count as it isn't HVAC controlled.
Having the
correct sized unit for your climate and Sq ft is very important. When you build a new house here, that is actually part of the inspection process. AC is something we spend $10,000+ on when building a new home here. Crazy, but it's a necessity. It's just baked into the construction costs/loans. Repairs or possible replacement can be a big factor when looking at buying homes with units nearing 10 years old. I know it factored into my decision to build new.
Then of course
preventative maintenance. My unit is 2.5 years old & I'll have maintenance done soon before summer hits here. I have 2 return air filters I change regularly myself. There is also a condensation drainage pipe in my attic central unit I ensure is clear and pour bleach down occasionally to clean out.
I have a
pre-programmed thermostat based on schedule. House temp goes up to 80 F when I'm working and is 76 F when I get home. I can adjust manually from there if needed. That automation based on my schedule is baked in savings.