Standing down staff indefinitely is not in line with Fair Work I believe. Stand down is only for a short-term stoppage of work beyond a company's control, not a lengthy downturn resulting in a long-term reduced schedule.
How long is short-term? It's complicated. Potentially up to 2.5 years, the training freeze? It could be said retraining crew would be cost prohibitive if the plan is to have those crew back on their original aircraft within that freeze period.
Or if it involves circumstances beyond the control of the company, one could say the government travel restrictions to various countries are an example of that, and crew will be stood down until those restrictions are removed and prior passenger loads return.