The Australian Constitution, unlike the American Constitution, does not actually give citizens positive rights. Instead, the Australian Constitution acts to limit the legislative power of the Government. Therefore, there is no "right to free speech" under the Australian Constitution. There is a semi-right to political speech. However, it operates as restricting the Government from legislating laws to prevent or suppress or limit political speech. It does not actually give citizens a right to say political speech. This is a nuanced but important difference.
In answer to your question, no, Facebook removing a post is not a contravention of free speech. Facebook is an American company, so firstly, you would be looking at the American Constitution and not the Australian Constitution. Secondly, Facebook has its own terms and conditions which each member agrees to before opening an account. If Facebook removed the post pursuant to their terms and conditions for breaching one of the terms then this is perfectly okay. You have agreed to it. Thirdly, the right to freedom of speech under the American Constitution is limited by other rights and laws. Therefore, if the post was offensive or discriminatory, then the First Amendment's freedom of speech will not protect it.