If your employer is instructing you to stay at home, then your employer should pay you as if you are at work. You cannot be made to take personal leave if you are not unfit for work because of an illness or injury.
You cannot be compelled to take annual leave without agreement. You may be directed to take excessive accrued annual leave, but this is unlikely in the case of an apprentice. If you do have excessive leave, then there are certain rules that apply before you can be forced to take it.
The Fair Work Act allows an employer to stand down employees if there is a ‘stoppage of work for which the employer cannot be held responsible’. A downturn in business does not automatically mean there are grounds for a stand down. If you are sent home, but your co-workers continue to work, that may not be a valid stand down, because there is still work to be done.
If the employer cannot have work done because of government orders, that may become a ‘stoppage’ but it will depend on the type of work, the nature of the government orders, and what the circumstances are.