How will the government modify our posts? 
Coersion, regulation and brute force.

" “Lawful interception” is a network technology that allows electronic surveillance of communications, as authorised by judicial or administrative order. There are standards (which means regulations and rules) for telecommunication and internet service providers to achieve this"

And if that does not work then they just hack you:

"Under the Identify and Disrupt Bill, access can be gained to encrypted data which could be copied, deleted, modified, and analysed...
they depend on “zero-day exploits”, which use software vulnerabilities that are unknown to software vendors or developers, to hack into a system. These vulnerabilities could be exploited for months or even years before they are patched. ... 
To protect citizens, we would expect these agencies to report or disclose any software vulnerabilities they discover to the software manufacturers so the weakness can be patched. However, they may instead choose not to report them and use the vulnerabilities for their own hacking."

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https://theconversation.com/facebook-or-twitter-posts-can-now-be-quietly-modified-by-the-government-under-new-surveillance-laws-167263