I think it's fair to say that national security is never defined with full elaboration in any of these statutes. It's mentioned, and in many cases it's up to the judge to determine whether something could be injurious to national security. That's a phrase that's commonly seen in law.
Some examples that might constitute national security would be something that's related to terrorism, espionage, counter-proliferation, subversion.... I'm drawing primarily from the way threats to the security of Canada are explained in the CSIS Act.