They might have been able to use the ADF, and with the benefit of hindsight they should have. But at the time the decision to use private security firms was made those firms were regarded as reputable business's which would deliver the service required. We now know that was not the case. Clearly things were pretty confused and the senior public servants organizing the quarantine were out of their depth. This was a unique situation for them and they were being asked to do things they had never had to do before, so they lacked any guidelines. Public servants without guidelines can and do make arbitrary decisions which may or may not turn out to be for the best.
