Australia is a long way from anywhere else in the world, so for most visitors the only practical way of getting into Australia is by air.
Approximately half of all travellers first arrive in Australia at Sydney's Kingsford Smith Airport, (IATA: SYD). Significant numbers of travellers also arrive in Australia in Melbourne, Brisbane and Perth. There are also direct international services into Adelaide, Cairns, Darwin, the Gold Coast and Christmas Island.
Sydney is a 3 hour flight from Auckland, New Zealand, a 7-11 hour flight from many countries in Asia, a 14 hour flight from the west of the United States and Canada, a 14 hour flight from Johannesburg, a 13-16 hour flight from South America, and up to a 24+ hour flight from western Europe. On account of long journey times from some destinations, travellers from Europe must have a stop-over, commonly in Singapore, Hong Kong, Dubai, Bangkok or Kuala Lumpur.
If you have to change to a domestic flight in a gateway city, Sydney, Brisbane and Perth all have distinct domestic terminals, requiring some time and complexity to transit: check the guides. Melbourne, Adelaide, Darwin, Cairns and the Gold Coast all have gates in the one terminal building or within easy walking distance of each other.
Australia's national carrier is Qantas, which together with its low cost subsidiary Jetstar operate many flights into Australia from all 6 inhabited continents of the world. Virgin Australia also flies several routes from North America and the Pacific islands into Australia. For those coming from Europe, Singapore Airlines and Hong Kong's Cathay Pacific make good alternatives to Qantas, British Airways or the Gulf airlines for flights into Australia. Some routes into Australia are operated by discount airlines such as AirAsia X, AirAsia Indonesia, Scoot, Tiger Airways and Jetstar Airways.
Cruise ships are available mostly in the November to February cruising season, and there are usually about 10 ships that arrive in Australia from other countries during this time. You can cruise to Australia, and then fly home. Holland America Line, Princess Cruises and Royal Caribbean all offer cruises to Australia across the Pacific.
Alternatively, you may sail to Australia in your own yacht, but beware of customs regulations. See Australian Customs for details.
There are no international ferry services operating.
By overland transport
There was a time when a couple of tour operators offered overland trips from London to Sydney, with only a short hop by air from South East Asia to North Western Australia while the bus went by barge. Currently, the only such tour operator is Madventure which runs 4 different routes: 26 weeks through Iran, Pakistan, and India; 26 weeks through the Caucasus & Central Asia; 64 weeks around Africa, the Middle East, & South Asia; and 64 weeks through Africa, the Middle East, the Caucasus, & Central Asia.
For those determined to travel overland as much as possible from Europe, you can travel independently to Singapore from Europe by train and/or bus on scheduled services, and fly from there to Perth (3,500 flight kilometres). For the truly determined overland traveller, you can get a ferry from Singapore to Indonesia and make your way across to Bali, where you can fly to Darwin (2,000 flight kilometres). For the intrepid, ferries to West Timor, a bus to Dili and a flight to Darwin will mean only 700 kilometres in the air.
(Thanks to WikiVoyage.org)