I believe this was an AIC some time ago but for now all I can find is this info taken directly from the CASA website:
Co-pilot means all flight time while serving in any piloting capacity other than as pilot in command.
Civil Aviation Safety Authority - Pilot Log Books
Seems pretty clear cut to me. It doesn't really matter how 'meaningful' you think sitting in the right hand seat of a SP aeroplane operating the radio or giving the PIC a break in the cruise is, I can't see how it would be anything other than legally logable co-pilot time (possibly illegal not to log it?).
It's just as correct logging co-pilot time in a C172 as it is in an A380 (despite the
slight workload difference
) if that's what you're doing as far as I can see.
Also lynchpin/MNII have it correct re: single/multi pilot aeroplane. That one is clearly defined.