"A threat to national security"

In September, American peace activist Scott Parkin was detained without charge and then deported from Australia on the grounds that he was "a threat to national security". So what exactly does this entail? Speaking before a Senate subcommittee last week, ASIO chief Paul O'Sullivan admitted that Parkin had not been involved in dangerous or violent protest activity in Australia - but that the assessment was based upon information on Parkin's activities while in the country. Given that those activities consisted of organising demonstrations, this seems to confirm the suspicion that Parkin was deported for entirely legal and peaceful protest activity - in other words, for saying things the government doesn't like. In a democracy which purports to value (but strangely does not legally protect) the freedom of speech, that is simply frightening.