First pathway: a long-term strategy
If the family is not constrained by funds, and the choice in favor of obtaining an education in Australia is made firmly and finally, the most reasonable thing is to enroll the child in one of the boarding schools. In principle, this can be done at any stage of school education. The undoubted advantages of this path are a "soft" entry into the Australian education system, a reliable guarantee of successful admission to university, and an early solution to the adaptation problem. The disadvantages include the need to part with the child for a long time and, of course, the high cost.
A compromise option would be to send a son or daughter to an Australian school for the last two years of secondary school. By this time, it is advisable to speak English well, although in schools oriented toward teaching children from abroad, additional language lessons are necessarily included in the program.
Second pathway: the golden mean
For those who believe that secondary education is still best obtained at home, and that one should go to Australia exclusively for a university diploma, they will have to include an extra item in their educational route – a one-year Foundation program at a pre-selected university. This year is intended to fill the "gap" between domestic (11 years) and Australian (12 years) secondary education. The faculty and future specialty should be decided in advance. Students of the preparatory department are trained in core subjects and at the same time their level of English proficiency is brought up to the required standard.
Advantages – lower costs and gradual familiarization with the realities of life in a foreign country. Disadvantages – the need to have a secondary school certificate with a high average score and fairly solid preliminary language training.
Third pathway: the "train" has not left yet
Did the decision to go study in Australia come to you when you had not only finished secondary school but also entered a domestic university? You have two options. The first is to try to transfer to an Australian university, possibly losing one year (the reason is the same notorious gap in the duration of secondary education). The second is to obtain a bachelor's or specialist degree at home and send documents to the chosen university for one of the master's programs.
The advantages of this path are obvious – you save a significant amount on tuition in Australia, and in the case of a master's degree you can even try to get a scholarship. But there are also disadvantages. You will no longer be able to radically change your specialty (although a wide range of specializations remains open to you). In addition, a future master's student will have to pass a rigorous competitive selection, in which your "trump cards" will be the presence of an IELTS test certificate and high academic performance (ideally, a "red diploma").