Is it illegal not to go to school in Australia?

Yes. It is illegal for children to not attend school in any form, but there are options, of course, besides formal classroom schooling.

Schooling is compulsory for students up to the age of sixteen in Australia. If a child does not attend school, they can be homeschooled, but they must be supervised by someone registered as a homeschooler. Children on remote properties in the outback are schooled via a distance education programme, using radios and computers.