English is the official language of Australia. In order to study there, a student must have a certain level of proficiency over the language. This has to be manifested/ proved to the authorities of both the university as well as the visa office.
The best way of improving your level of English is by sitting the International English language Testing System (IELTS) exam. Most university websites give the level required to be achieved for the course the student intends to study in terms of the IELTS Score.
Other Tests: Higher studies in specific or professional areas such as teacher training or teacher assistant courses may require course specific tests as well.