Not withstanding the references mentioned above, CAO 40.1.0 para 10.5 details when co-pilot time can be logged.
The relief for the ATPL requirement for a single pilot certified aircraft ie. Metro comes from CAR 5.105. Then comes down to who can log what.
Dare say CAR 5.105 will receive an amendment at some point to stop CPL's flying aircraft heavier than 5700kg's in commercial pax carrying ops. Apparently CASA hate it.
D