Not sure it’s as simple as you suggest, JT.
For the period an Australian aircraft (VH-) is flown on MIL OPS by an ADF pilot it is considered to be a State aircraft.
Quite so (although the criterion is command in the course of duties, rather than ‘MIL OPS’, and it’s not ‘considered’ to be a state aircraft: it is, by definition, a state aircraft).
By definition, it makes no difference whether it’s got VH painted on the tail or has ‘DGTA registration’.
The question whether the aircraft is a ‘state aircraft’ is the horse; the applicable rules are the cart. In other words, you have to first work out whether an aircraft is or is not a state aircraft, in order to go on to work out whether the aircraft is DGTA’s responsibility or CASA’s responsibility.
Unless things have changed a lot over the past few years ….
Now that you mention it, here’s what the definition used to say:
state aircraft means:
(a) aircraft of any part of the Defence Force (including any aircraft that is commanded by a member of that Force in the course of duties as such a member), other than any aircraft that by virtue of registration under the regulations is an Australian aircraft; and
(b) aircraft used in the military, customs or police services of a foreign country.
[bolding added]
Note the profound difference between the previous definition and the current definition: Under the previous definition, a VH-registered aircraft was, by definition,
never a state aircraft. Under the current definition, an aircraft is
always a state aircraft when it is commanded by a member of the ADF in the course of duties as a member,
even if the aircraft is VH-registered (or has no registration at all).
So let’s take a very simple hypothetical. A CT-4 with ‘VH’ on the tail is only ever flown by PICs that are ADF trainees or ADF instructors, in the course of their duties as ADF members. Which regulator is responsible for the airworthiness and maintenance management of that aircraft?