I havent heard of this. Your local MRA may have some information that is up to date, as they have been heavily into the helmet debacle.
If the police did do this, then they would have to be able to state/show where it is laid down. It may be in the NSW rules, or they may have made it up... The average cop, has about as much knowledge of the helmet laws/rules/standards as we do, maybe even less.
I do know that it is recommended that they be replaced every 5 years.
Its not so long ago, that WA stopped checking helmets for compliance, as Standards Aust doesnt exist as such any more, and hasnt issued stickers for many years.