Have a look at the actual letter from the department.
It is the custom in letters from Commonwealth government agencies
that they state brief, basic reasons, and explain if, and how, you can appeal a decision.
There will be a timeframe (x number of days) in which this can occur,
which is basically not flexible.
If a person does not appeal, and does not leave within 27 days,
then they can be taken into custody,
and kept locked up until they are deported.
It is important to remember that people who help hide them, or who lie about where they are,
can be committing offences themselves.