Screening is required for all people who intend to board a designated flight that requires screened passengers, as other countries schema are not covered under Australian law (Aviation Transport Security Regulations 2005 ), so passengers are considered by default to be unscreened despite what treatment they may have received overseas. Even passengers from another Australian port who are transiting internationally are screened.
Thanks markis10 for your very succinct statement of what the policy is, but it does not seem to me to be sensible or rational. Why not have these provisions in place to use if it is known that screening back along the line was of dubious quality, but waive it if it is assessed that the other country's screening standards are comparable to Australia's?
Cocitus23.