For domestic travel in Australia, infants can fly free of charge up to their second birthday. Once they turn two, children are charged the full fare.
If you’re travelling with two infants, say twins, you usually need to have two accompanying adults. Some airlines allow you to travel alone with two infants but insist that you buy an extra adult seat onto which you fit an approved child restraint (see below); you’d have one baby in the seat and one on your lap. Specific regulations and requirements are listed on each carrier’s website.
For international travel, infants (up to two years) are charged 10% of the adult fare, but do not receive a baggage allowance. One they turn two, they are charged 75% of the adult fare (in most cases), and at 12 the adult fares kick in. There are variations to this, so check with the airline (or an agent) when booking. Virgin, for example, charges 50-75% per child under 12 on selected long-haul international routes to North America, the United Arab Emirates and Europe.