Electricity in sydney is about 29c/kwh
NG is 4.1c/MJ
9kg bottle are... $25 to fill? Thats 417MJ, so 17c/MJ.
Going for maximum efficiency, the Daikin Ururu Sarara is 580% efficient (3.6kw of heat for 620w power draw), so 5.1c per kw/h electric
The Paloma prs-250 heater uses 25mj/h of gas to provide 5.9kw of heat, so 17c per kw/h NG and 72c/kwh LPG.
Gas has gone up since I last worked it out- and LPG is pricey. I think you can get larger bottles for more like $1.50/L, which is more like 5c/MJ- pretty similar to NG.
Of course, if you have a smart meter then electric could cost more then the above in peak hours- 52c/kwh comes to 9c per kw/h of heat in peak with the above aircon unit.