The new law applies to cars that come from anywhere outside Greece, whether from European Union member states or further abroad. Then they must either be re-exported or sealed by the customs authorities. To circulate again after six months the precondition is that the car's owner was abroad for those six months. There are no exceptions. During the six months that the car circulates legally its owner is obliged to pay road tax.
Current permits are valid.
Because of Greece's membership of the European Union, though, people bringing their cars into the country are not obliged to declare them at the point of entry if they are arriving from an EU country - which in this case is usually Italy. So, theoretically, it will be very difficult for traffic police, customs authorities or the economic police to know when a car first entered the country. Therefore the onus will now be on the driver to prove that he is not a resident of Greece and that his car has not been in the country for longer than six months. In short, the driver of a car with foreign plates will be expected to be able to show the ship's ticket with which he brought the car here. If he is working in Greece he does not have the right to drive a car with foreign plates, so he will also be obliged to show his two most recent annual tax statements from the country in which he is resident.