Of course that doesn't mean we can't question our authorities, but where intelligence is involved it's extremely difficult to do so accurately because VERY little will ever be made public. As boring as it may sound - the government can't and don't control every little action of Police and our other agencies for their own political benefit.
They can control the public release of information about such threats/arrests and the following reaction.
In this case, it appears that if the AU Government didn't act publicly, the UK Government would.
There is a difference between making any necessary arrests and nothing more, saying "there was a threat, it has been prevented, carry on as normal" and turning the whole thing into a big show and increasing "security" in a way that ultimately does nothing more then increase the paranoia level and creating easier targets while doing nothing of any real use.
Liquid limits were put in place as a direct result of a "plot" that never got into operational planning and was not physical possible even it it had be tried.
Body scanners, which don't work, are easily bypassed and have excessively high false positive scans, were rolled out as a direct result of an attempt which didn't work whose perpetrator was known to authorities, had been warned about by family members, didn't have a visa and was still let on the flight anyway.
Shoes are required to come off at checkpoints in many parts of the world as a direct result of an attempt which also didn't work whose perpetrator was again known to authorities and had been stopped and turned away at the checkpoint. Where upon he returned the next day and was guided around the screening.
There is also the US government making threats about "do this at screening, or we'll ban large electronics on flights to/from US".
While there is some level of threat, such threat is often overblown by authorities. Even though authorities can't (always) control such actions, they can and do control the response to said actions. Such response is usually unwarranted and often far more damaging then the thing they prevented would have been.