According to the University of South Australia site the apostrophe is included, but according to the Australian Government's style guide it isn't included.
I consider this to be the situation where our language is evolving and the apostrophe is no longer needed. Interestingly Microsoft's grammar checker suggests the apostrophe should be included. If the language has evolved then the suggestion by Microsoft's grammar checker is now wrong and misleading people. I've seen quite a few examples of where the spellcheckers suggest incorrect words so this isn't a total surprise.
What do you think?