Know that most individual universities in Australia have their own set of criteria for admissions. While gap year acceptability varies from one university to another, small gaps of one or two years are normal. You will however need to justify larger gaps of three to four years though.
Other factors for gap acceptability depend upon whether you want to join the undergraduate course or the postgraduate course.
Justification is necessary though irrespective of the course you intend to join. You may also have to provide evidence as a reason in some cases.