Why would it be questionable?
It's an encroachment on the division of powers in the constitution. Certain things are unambiguously reserved for the Executive Government, some things reserved for the Legislative Government, and some things are exclusive to the Judicial Government.
Among the things under the Judiciary head of power is the ability to hold trials and determinations of fact/guilt. Tribunals are run by the executive arm, usually as part of a governing/specialist body for a particular field. For instance, the Industrial Relations Act (created by Parliament) creates an Industrial Relations Commission (part of the executive), which can hold tribunals on Industrial Relations issues without them needing to go to court (the Judiciary). Tribunals or tribunal bodies are typically set up by the legislature in such a way that they are subordinate to the judiciary (e.g. the decision of a tribunal can be appealed to the higher court), or by the judiciary otherwise according to delegate some level of their power to the tribunal body.
In the first case, then the judiciary has a supervisory capacity anyway, which means that the standard rules governing due process etc. still typically apply. In the latter, the court can't delegate power it doesn't have, so you still have due process rules etc.
Military tribunals are part of the executive government, as all military things are. The issue with military tribunals is that they often try to violate various procedural requirements that normally operate in the judiciary. A common example is that they try and keep them confidential so the judiciary can't supervise, but that often involves creating procedural disadvantages. e.g. in the David Hicks case, he was charged with offences, but wasn't told what they were because they would compromise national security. This meant he couldn't mount a defence.
The reason it's questionable is because it's not clear that the Executive government are actually given the power in the Constitution to operate in this way, because they are usurping the role of the judiciary.