Michael O'Brien responded to my email with the following

Dear Chris,

As I understand the letter, it is making the point that the measured noise
levels (before the recent works) in your area are not in excess of the
VicRoads decibel limit for which additional noise works are required.

That is to say, the recent works were done purely as a trial and not
because your area's noise levels meet the threshold for required
enhancements under the state noise attenuation policy.

I read the letter to say that the before and after noise measurements will
be used to determine whether the recently performed works remain, or are
modified in some way.

That is, I believe that what you understand to mean "the existing noise
barriers" as including the recent works is different to what VicRoads
intends (being the original works prior to the recent modifications).

As a consequence, I understand that VicRoads will be guided by the before
and after noise measurements in determining the future of the recent works
- as we previously discussed.

I trust this assists.

His response totally ignores the point I was trying to make. It's like watching a politician being interviewed on TV where he is asked one question and gives his answer about completely unrelated!