There are basic aspects of travel to consider. More daylight is better than less (usually). Warmer is better than colder most of the time – with exceptions for deserts. Outside of the main tourist season or school holidays is better – unless you really like crowds. We considered all of these factors in our Australia trip but still suffered adverse conditions at times.
Northern Australia and the central desert is hot during the middle of summer. The area of Darwin and far north is hot for most of the year, and only tolerable for many people during the height of winter. However the mid-winter period is too cold and rainy/cloudy in southern Australia at this time. So if both locations are to be visited some compromises are inevitable.
The Bureau of Meteorology has a detailed website with both forecasts and basic climatological data to help plan your trip. Maps of averages of many quantities can be found here. However, it is very important to recognize that climatological averages are not what you will see on a daily basis. Some days will be well above average and others will be much colder than the monthly averages indicate. The BOM has a simplified explanation of some of the atmospheric features that affect Australia at various times of the year.