The Bureau of Meteorology has forecast cold weather for the South East, wet weather for the North and East and heatwaves for Western Australia on Boxing Day.
Sydney is expected to remain cloudy in parts, with a low of 50 degrees and a high of 50 degrees in part.
Regardless of whether it comes to that, Mr How said people were told to look out for cyclones and consider preparing for them.
Hobart is also likely to have a cool Christmas with a minimum of 15C, a maximum of 21C and fair cloudy sky.
The forecast is mostly cloudy with a mild high of 20C.
Meanwhile, Melbourne is predicted to show a relatively cool day with at least 16C and max 21C.