Best time to visit tropics like Darwin, Kakadu National Park or northern Queensland is in dry season - best weather, no rain
Best time to visit Perth - Spring or Autumn. In summer temperatures can go up to 40 degrees and drop in winter to 5 degrees.
Best time to visit Alice Springs - Spring or Autumn - same reasons
Best time to visit Adelaide and Melbourne - summer; sometimes temperature can go up to 45 degrees, but that's just happens once or twice in summer.
Best time to visit Sydney or Brisbane - ANY time - all seasons weather is perfect for sightseeing.
Check out my blog of how we travelled around Australia - https://www.travelspicedlife.com/