Jay, you can never say categorically "extremism", and its associated violence, is dead and buried and will never raise its ugly head again in any society, even if that society does not have a tradition of such extremists behavior, like Australia. Where ever extremism has come to the fore its been the result of some kind of massive social dislocation and inequalities, Russia and Germany last century. Given the right conditions things can change rapidly, but even so if conditions are conducive for the rise of violent extremism the vast majority, although maybe agreeing to the underlying principles of the extremists do not become directly involved or necessarily condone the extremists violent action.

<< The fear campaign being waged by the Abbott-Shorten government and their sympathetic journalists is unjustified, there really were Reds under the beds just as there really are Jihadis in getting about but they're clearly not determined to carry out a "spectacular".>>

Prior to WWII, and there shortly afterwards, communism was seen in a somewhat different light by the majority in Australia. Conditions in the 1930's were far more conducive to extremism, with about 30% unemployed, than they were post WWII. It was also difficult for conservative politicians to demonise communists, because most people (workers) could not see any wrong that had done by them, and thought they were even a positive for workers. Working within the unions to better workers conditions etc. Post WWII the conservatives, lead by America, were successful in changing society's attitude through a well orchestrated fear campaign against communists and communism, "The Russians are coming." The ruling elite always need a negative to hang their hat on, be it communists, jihadests, whatever, they need some kind of distraction, real or imaginary, for the underclass to focus on, some group other than themselves. "See 'they' are the cause of your problems (underclass), and I will work, for you, to overcome the problem." Basically that is how the elite present the argument.
I agree with what you say.

I think what is being overlooked here is that the current worries are about
groups that have a tradition of car bombs.
We are not worrying about the odd demo that results in a bit of pushing and shoving.
They could be now establishing a practise of murder by hit & run.