I don't know where you are located in Australia or what your winter averages are (0-5C lows and 10-15C highs???). I would keep all of the temperate species out of the greenhouse and in a sheltered location, like a cold frame or something. I definitely wouldn't want temperate trees in a spot where they get less winter chill than your environment provides (such as in that greenhouse). Especially since in most areas of Southern AU, the chill would be just barely adequate for many temperate species as it is.
Trees such as Japanese maple and Japanese black pine need a certain number of chill hours to break dormancy properly and resume growth. If fully dormant and in a sheltered wind protected location, these trees can take temperatures down to at least -5C without issue (in many cases far lower depending upon the species, but that should not be an issue for you unless you live in the high elevation areas of the extreme south). Allow the temperate trees to properly achieve dormancy. You have plenty of growing season for them without using the greenhouse to extend/interfere with it.
