This particular paragraph in fact says to you that it's “not a defence” that “the activity was authorized by an Act or a regulation or other statutory instrument, unless the defendant proves”—unless you prove—“that the significant environmental harm is or was the inevitable result of” doing a legal activity.
Isn't that a defence? What you're saying is that you don't have a defence, that it removes the defence. I'm suggesting to you it doesn't remove the defence; in fact, it reverses the onus for you to prove that you were within your rights.