Never thought mosman was bad. Just dont cross paths with Kuringai Highway Patrol.
As said, some officers take it upon themselves to be bastards, theres nothing you can do about it unfortunately. Regarding your exhaust I highly doubt its under 90db.
If you are pulled over and hassled when you havent committed an offence, feel free to ask for the officers badge number. If he doesnt give it to you, demand it. If he still refuses to give it to you hes in alot of trouble, write down his rego and head straight to the nearest police station.
I doubt its a fine for having the exhaust on your car period, more so if its not ADR compliant.
I hear of these idle threats of defects and fines in the mail regularly and most of the time it never happens, more so just the officer acting high and mighty.
Dont stress about it