If the correspondence reveals information which is to do with 1. the prevention or detection of a crime, 2. the apprehension or prosecution of offenders 3. the assessment or collection of any tax or duty or of any imposition of a similar nature,
then the DP Act doesn't apply. Point 3 certainly seems to apply in this case.
As for revealing any information, you don't have to in cases such as this. Just reply 'not known'.
I used to have dealings with the DP Act and had to decode a version of the whole act in plain English for the company I worked for. Not fun!