Justiciar -
of which 15 have to be with an instructor, ie the balance of 25 hours is flight time not instructional time?
Correct.
mm_flynn -
the 15 hours must be with a 'proper IR instructor'
That's a perfectly fair personal interpretation - as are others.
The FARs are silent on this weird combination of US and non-US instrument privileges...
One soon gets into debating the meaning of "authorised instructor" which has never been settled by the FAA in the context of a non-FAA instrument qualification (like the UK or Australian sub-ICAO one). Some commentators have interpreted this as "ICAO IR instructor" (presumably because the FAA IR is an ICAO IR) but the FARs don't say this, either.
I have no idea if Australian pilots have forums too
but if they have, this is sure to be a topic on there too, for those going to the USA to do the FAA IR.
Anyway, this one is definitely within the discretion of the individual DPE doing that specific checkride. As are a pile of other things.