I think you can only log co-pilot time if acting as a co-pilot in an aircraft where either:
1. you are operating as co-pilot in an aircraft that requires at least 2 pilots (according to the flight manual) or
2. you are operating as co-pilot under an AOC that specifies the aircraft will be operated as a multi pilot aircraft.
I think there is (suprise, suprise!) an unintended discrepency in the part 61 stuff (see 61.460 Privileges of recreational pilot licences)
that says an RPL *may* act as pilot *or co-pilot* in an aircraft that is certified for single pilot operations so - if the 180 is operating under an AOC and the AOC requires it be flown as a multicrew aircraft you might be able to!
At least until someone in CASA notices the typo!