Hi Tasna,
There is a limitation period in NSW. For a parking fine it is 12 months. The limitation period means that the fine needs to be issued for a parking offence within 12 months.
Once the fine is issued, if not paid, it can be "enforced". Then the fine lives forever until paid/resolved. It looks like this is your situation.
It is quite possible for you to see this enforcement for the first time. If the original fine was left on your car when someone drove it then she could have just thrown the fine away. You would get a reminder notice however it will go to your RMS registered address. If you have not changed it and lived in a different place to what RMS knows then you would not get the reminder. The next step is the enforcement...
You can dispute the fine in court. You ask State Debt Recovery for an annulment of the enforcement order. If granted, you get your day in court to dispute the fine.
Good luck.