What is the hardest degree to get into Australia?

Could you elaborate on what makes a particular degree program in Australia considered the hardest to gain admission into? Are there specific criteria such as academic performance, entrance exams, or competition among applicants that make it particularly challenging? Additionally, are there any particular fields or disciplines that are known to have stricter admission requirements than others? Understanding these factors would help provide a clearer picture of what makes a degree program in Australia difficult to enter.