A lot of people would like to include their partner on their student visa. Being able to have your partner join you on to study in Australia will mean you can stay together. It is also a lot cheaper to only pay for one person to study in Australia.
To be able to include your partner as a secondary applicant on your student visa, you must be classified as a member of the family unit of the main applicant. To be assessed as a member of the family unit you must be the spouse (husband or wife), or the de facto partner.
To meet the de facto requirement you must prove that:
Some of the requirements that immigration will jude your de facto relationship include:
To have your visa approved, you must demonstrate that you meet the defacto relationship for at least 12 months before the application is made.
Source: http://pathwaytoaus.com/blog/can-i-include-my-partner-on-my-student-visa/