I did a bit more research and apparently, AS16898 has much more stringent penetration tests that the Snell standard, so whilst OS models may look the same, and have the same name etc, they may actually be different to meet the more stringent AS1698. This may mean that they are actually a bit heavier than OS models.
The real risk to using a non-compliant helmet would be post accident, insurance companies will probably find out after the investigation, and I know I would'nt want to be in that situation. Copping a fine is nothing in comparison, but overall, I wouldn't take the risk, as tempting as it is. Brad is also correct about the liner label.
regards