Short answer
Most students cannot work before their course starts. While the course is in session, the usual limit is 48 hours per fortnight, unless an exception applies.
Most students cannot work in Australia before their course starts. An exception may apply if they held a visa that allowed work when they applied.
While the course is in session, students usually cannot work more than 48 hours per fortnight. A fortnight means a 14-day period starting on a Monday.
Work that is a required part of the course may be treated differently. When the course is not in session, students may be able to work unlimited hours.