Hello,
It depends on whether or not your running under CAA or CASA rules. CAA rules stipulate that if the aircraft is run under two crew operations, and the Captain is trained for com. practise, and you the pilot are responsible for the operation of the aircraft (under supervision) then yes it's perfectly legal. The only difference CASA has (that i'm aware) is you need a command rating on the aircraft that you wish to log the command practise.
Hope this helps