So DHA wants tech companies to provide the government access to the contents of encrypted communications. -- I'm not sure how this can be done without putting inadvertent vulnerabilities in.

Maybe a solution is for tech companies just pass metadata to law enforcement, similar to the way telecommunications companies do. The tech companies could be put in the same bucket at telco companies and be subject to Australia's data retention obligations. https://www.homeaffairs.gov.au/about-us/our-portfolios/national-security/lawful-access-telecommunications/data-retention-obligations