Your gas usage is metered in cubic metres. A cubic metre of natural gas (mostly methane) usually produces about 38 MJ of energy for you when it is burned, in standard conditions. Natural gas can contain up to 7% of other gases that are not burned (eg a bit of carbon dioxide and a bit of nitrogen), effectively non-fuel. But the AEMO assumes it's more typically around 3%. So the heating factor isn't something that can be changed or lowered (except by natural variation in the gas source itself), think of it as a conversion factor between volume and energy. Standard conditions are 101.325 kPa and 15 degC for gas, but actual conditions will be slightly different, hence an additional conversion or correction factor. Hope that makes it clearer!
For those with a technical interest, the Jemena Eastern Gas Pipeline Maintenance Manual (61p) has all the definitions. https://jemena.com.au/documents/pipeline/egp-measurement-manual.aspx
The AEMO also has a table of correction factors for varying pressures: https://www.aemo.com.au/-/media/files/gas/retail_markets_and_metering/market-procedures/vic/2017/aem... - Note that these are pressures additional to standard 101.325 kPa (1013.25 millibars for those that remember old weather reports!)
Just a further thought: To lower your gas bill, what you need to do is lower the actual number of cubic metres of gas used (you can see this on the meter near your gas tap, usually on the side of the house). You might be able to track down whether the heating or hot water is using the most gas by writing down the number on the meter each day for a couple of days when you use the heating, and each day for a couple of days when it's cold but not quite cold enough to turn on the ducted heating. The ducted heating is likely to be the larger user. (We found this - we get months of heavy frosts!) Check how much insulation is in the roof - it pays off over time to top it up and it's more comfortable. If the hot water tank is outside, and it's old, it might be losing a fair bit of heat too. Might be time to look at replacement options. (Sorry these solutions involve spending, but will save in the long term! And if you're renting, options are limited...)