Australia is roughly the size of the contiguous United States. Internal flights are essential - driving Sydney to Cairns is 30+ hours and rarely worth it on a holiday. Plan one major destination per 3-4 days.
7 days: Sydney + Blue Mountains, or Melbourne + Great Ocean Road, or Cairns + Great Barrier Reef.
14 days: Sydney → Uluru → Cairns/Reef, or East Coast Sydney to Cairns with Whitsundays sailing.
21+ days: Add Western Australia (Perth + Margaret River), Tasmania, or a New Zealand extension across the Tasman.