Why Australia
The country is twenty-four times the size of France for twenty-seven million people, almost all of them on a coastal strip. That explains everything else: the cities are far from one another, the interior is empty, and you do not "do Australia" in three weeks.
The real choice is between two countries. The east coast — Sydney, Brisbane, the Gold Coast, Cairns — holds the population, the beaches and the Great Barrier Reef. The rest, from the Red Centre to Tasmania by way of Perth, takes time and domestic flights.
Getting around
Flying is not a luxury but the norm: Sydney–Perth is a five-hour flight, which is more than Paris–Cairo. Low-cost carriers cover the main routes well, and a domestic flight booked in advance often costs less than a train in Europe.
On the ground, a car is necessary as soon as you leave the city centres. The campervan is a way of travelling in its own right, with a well-documented network of free sites. The distances between gas stations, in the outback, are checked before you set off: several hundred kilometres with nothing is a common situation, not an exceptional one.
Money
Everything is paid by card, including three-dollar coffees and the markets. Cash has become rare to the point that some businesses refuse it. Plan on a card with no foreign transaction fees rather than on cash.
Tipping does not exist: the minimum wage is high and the displayed prices are the prices paid. A bill has no surcharge, and rounding up is neither expected nor frowned upon.
Before you go
The sun is the first risk and it is underestimated. The southern ozone hole makes the UV index extreme even in overcast weather: Australians wear hats and long sleeves, and that is not excessive caution.
Biosecurity checks on arrival are strict and the fines heavy: any food, plant product, wood or muddy hiking boot must be declared. Declaring costs nothing, not declaring costs a great deal.
Road distances are read in hours, not kilometres, and fatigue at the wheel kills more than the wildlife. Kangaroos come out at dawn and dusk: Australians do not drive at night outside the cities, and it is a habit worth adopting.