wa police concealed number plate fines

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I know in ACT its not concealed, its registration plate not attached/incorrectly attached, and then it goes into the modified bits as well. But as far as I know if I can't read it, it'd be fined. I've seen Police fine muddy 4x4's, espically if you can't rub it off or its muddy under a plastic guard. With traffic infringement there is so much discretion that cops can ping or not ping to their hearts content.