The roof contributes 25–35% of total heat loss or gain. Add ceiling insulation and sarking. Bunnings now stocks R6 insulation which is fantastic, or get a professional to install it. If you install it yourself, ensure all power in the house is off — there is an unfortunate history of fatalities from amateur roof insulators. If you can only insulate one thing, insulate the roof.
Walls contribute 15–25% of total heat loss or gain. Even brick walls can have insulation pumped into them. If you are using pump-in insulation, get the electrician to do their work first (unless they want to be showered in insulation) and seal any gaps first.
The floor contributes 10–20% of total heat loss or gain. Use a hydrophobic material with as high an R value as possible.
If you can afford it, replace single-glazed windows with double-glazed. If the budget is tighter, consider alternatives: external shutters, internal double-cell honeycomb blinds, a pelmet above existing curtains to trap heat, a thermally efficient window film, or installing a second set of windows to make your own double glazing.
Look for a low U-value with a thermally broken frame, a timber frame, or a UPVC frame.
A reverse-cycle air conditioner is generally the most energy-efficient option for heating and cooling. Help it along with ceiling fans in each room. If you can install underfloor air conditioning, it is generally even more efficient.
Look for a heat pump hot water heater and set it to heat water when you are generating solar power.
Replace lighting with LEDs — they are more energy efficient. Look for lighting with a CRI of 95 or above so the colour rendering is as close to natural sunlight as possible.
When looking at houses, a basic thing to get right is to find a north-facing house. North orientation lets you get sun in winter and keep it out in summer with the right overhangs.