The best time to visit Australia
Australia is not one climate, it is at least four. The honest answer to “when should I go?” is “it depends which Australia you mean.”
The short answer
If you want a single window that works reasonably well for a coast-to-coast trip, aim for April–May or September–October. The southern cities are pleasant, the Red Centre is comfortable rather than brutal, and the Top End is either just out of, or just heading into, the dry season.
Region by region
Sydney, Melbourne & the south coast
October to April. Summer here runs December to February with temperatures in the high twenties and long light evenings. The Great Ocean Road is at its best in this window, though it can be windy year-round.
The Red Centre — Uluru and Alice Springs
April to October. Summer in the desert regularly clears 40°C, which makes the sunrise walk considerably less romantic. In the cooler months you get warm days, cold nights and the clearest star fields you will ever stand under.
The Top End — Darwin, Kakadu, Katherine
May to September, without much debate. This is the dry season: roads open, cruises running, humidity manageable. The wet season (November to April) brings spectacular storms and flooded access roads in roughly equal measure.
Tropical North Queensland & the Great Barrier Reef
June to October. Calm seas, the best underwater visibility of the year, and outside the stinger season, which runs roughly November to May.
New Zealand
November to April. The South Island's alpine scenery needs long days to be seen properly, and Milford Sound is at its most reliable in the southern summer — though it rains there around 200 days a year regardless, which is precisely why the waterfalls are so good.
Why this matters less on an escorted tour
A well-designed multi-country itinerary sequences its regions so that you hit each one in a sensible season — southern cities early, Top End in the dry, reef when the water is clear. That routing is quietly one of the most valuable things you are buying. Working it out yourself, with six internal flights to book around it, is a genuine project.