The media (and many others) have totally lost the plot and don't seem to have a brain to share between themselves.
Chasing down the exact person who decided on private security guards is totally & utterly missing the point. Private security guards have been used in at least three states of Australia and others places overseas for that exact role and that decision is not what caused the problem.
The execution of hotel quarantine and the overall processes are what resulted in us being where we are. Who was allocated the job of managing the program ensuring contract conditions were being met. Who was the project manager reviewing where the "project" was at, and fine tuning the process so it operated effectively and smoothly. Who wrote the procedures to be followed, the precautions to be taken.
Whether state police, federal police, ADF, private security guards, prison wardens, or kindergarten teachers were used is totally irrelevant to making sure it doesn't happen again. None of the options that have been alluded to by various people with regard of the source of "guarding" are trained for an infectious medical containment process. Not even nurses in our hospitals were fully au fait on what to do until they received relevant training.
Anyone, regardless if from from your local high school kids through to a group of high ranking officers from the ADF would need to have been properly kitted out and trained on what to do and monitored - and that didn't happen. Whoever decided on what resource to use and sign the contract has buggerall to do with what we actually need to know.