Once your code contains GPL code that is not yours, it has to be GPL. Particularly in v3, where a number of loopholes were closed. You are free to attach further non-conflicting clauses to it, but the GPL of the original code must be respected. That’s the entire point of the GPL.
Double-licensing requires you to have ownership of the entire codebase. At that point, you are licensing everything, so you’re free to pick any license that suits you.
But to be honest, as an Aussie I don't think our government is remotely competent enough to have considered the copyright license of the code they were using. There were initial reports they would provide the source code of the application, but these promises were quickly revoked for reasons of "national security" or some other such rubbish.
EDIT: I meant to say that it was a bullshit reason such as "national security", not that it was a direct quote. The actual reason they claimed was that it was easier to hack if the source code was public.
Were they? Where did you see that?
The Health Dept's response to the Privacy Impact Assessment's recommendation for release of the app's source code says as follows:
Agreed. The PIA and source code will be released subject to consultation with the Australian Signals Directorate’s Australian Cyber Security Centre
(https://www.health.gov.au/resources/publications/covidsafe-a...)
Now that 'consultation' might be a delaying tactic, but it's just as likely to be that the Dept. of Health has no idea of the implications of such release. It certainly doesn't suggest the revocation you claim.
Now, it's very possible that they'll release it tomorrow and this whole discussion will have been a waste of time -- but at the time of writing the Minister for Government Services said unequivocally that they would release the source code[2] and later the Health Minister said they were "unsure it would be safe"[1], and finally when the app was released the source code was nowhere to be seen. To be fair, he was insistent that they would release it (despite being "unsure it would be safe").
But sure, I also wouldn't be surprised to discover that the whole process has been delayed by some other bureaucracy. After all, they probably see releasing the source code as a token gesture and not a form of review by the public.
[1]: https://www.itnews.com.au/news/health-minister-now-unsure-if... [2]: https://www.itnews.com.au/news/govt-to-release-source-code-o...
[1]: https://www.itnews.com.au/news/health-minister-now-unsure-if...
“The first thing we want to do is make sure that we're protecting the safety and the privacy of individuals. Everything that can be released, will be, for sure,”
I don’t share your cynicism but I do hope that “everything” means everything.
OP meant Singapore can dual license.