hellonearthisman: This law is a badly written law, 1st it's guilt on accusation and 2nd yes you can get a notice dated 21 days before the law came into effect.
The 21 days is irrelevant, the bad part of the law is 122N where it is up to you to prove you didn't do it. I think the rights owners should have to produce more evidence, such as proof of the methods they followed to identify you as sharing copyrighted material.
The way the law is written at the moment the only way to prove your innocence would be, for example, if your internet was broken at that time.
It's also $25 + GST per notice, not $20.
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