Perhaps the Aussie requirements are different, on this side of the ditch an ATPL allows you to be P in C of an aircraft where the flight manual requires two pilots.
If there is no requirement for two pilots in the flight manual then all that's required is a CPL.
For example situations like hire and reward IFR without an operative autopilot in something like a Cheiftain, require two pilots to operate the aircraft but can the done on a CPL since the aircraft flight manual doesn't require two pilots to operate the aircraft.
Who or what is driving the requirement for an ATPL on single pilot RPT ops? Seems to be an overkill to me.