I mean, you're pretty much advocating for the rough equivalent of a warrant canary, which have already been explicitly outlawed in Australia. [0]
[0]: https://boingboing.net/2015/03/26/australia-outlaws-warrant-...
[0]: https://boingboing.net/2015/03/26/australia-outlaws-warrant-...
The Assistance and Access Act does have the same "existence or nonexistence" wording as the law you referred to, so you're not wrong that a warrant canary for a notice under the new legislation would be a criminal offense. But there is no blanket law against warrant canaries per se, it's done explicitly in each new law about secret warrants.