So in my experience, over the past year UQ has set deadlines for us (the class entering year 2 now) to arrive in Australia, and has held firmly to those deadlines saying they cannot be moved until it becomes clear they will be impossible to meet. At which point they pushed the deadline a few months.
On the other hand, there were a handful of students in the year above me who chose to return home when the pandemic hit (from what I've heard, relying on advice from UQ at the time that returning home was okay) and were unable to reenter Australia. Those students were required to defer a year.
So I think it really depends on how many students are in the given situation. In the first situation, requiring nearly my entire class to defer would have caused huge issues running the program, affecting not only the current class but also future classes. The second situation was far fewer students and didn't cause much of a disruption (except to those students affected). If OzTREKK thinks enough students are affected, they're probably right--they've been working with UQ admin for years. But I really do hope things work out for you without requiring an extension, it is definitely a bit tight but there is still time!