DoS doesn't have to be intentional. It can happen merely because too many people, through their PCs, try to access a site all at once.
If the equivalent of short-term Random Access Memory (RAM) is so configured for a "Mainframe" that is not programed to accept so many users all at once, then the Mainframe "Denies" to "Serve" any of the users. It is perceived to "crash".
Users in locations all over Australia automatically go through US-based servers (eg. at *) to access sites back in Australia. That is why people at internet security agencies in Canberra would have perceived a spike in traffic (around 7.30pm #) from the US to the Census site in Australia.
# A particular problem I saw on the night was that after the 7pm TV news reminded poeple to fill out their internet Census a high simultaneous number of Aussies tried.
When tried to access the Census site, I saw that the Census site was not simply Crashing at my first attempt (first 10 seconds or so) which would have "cleared the deck" for 100,000s lucky users. Instead the site was allowing 10 to 20 automatic "redials" every 10 seconds or so.
If there was millions of Aussies trying around 7.30pm then that would therefore register as up to X? x 1,000,000s x 20 near simultaneous attempts - which would Crash almost any poorly planned Mainframe RAM.
* many US servers eg. Google and Microsoft, are at Mountain View, Sunnyvale, California http://en.wikipedia.org/wiki/Mountain_View,_California#Top_employers serve internet users from all over the world
So the ABS Census Mainframe couldn't handle what was perceived to be, maybe, 5,000,000 x 20 = One Hundred Million near simultaneous users at 7.30pm on 9 Aug 2016.
Shows how a Technical Problem becomes a Political Problem for Australia's Prime Minister Turnbull, already governing on a knife-edge.