When can you call yourself doctor?
Writing as an Administrator: 

It is appropriate to use the title when you are a graduate, ie, when the degree is conferred either in notice by letter or by ceremony (which ever comes first).  Prior to that your status is that of a graduand.  If you've been using the work-title PhD Candidate you might consider changing to PhD Graduand to indicate this status: that you're awaiting conferral but you've met the substantive criteria for fulfillment of your degree.  Additional source: Swinburn on Postnominals.

Congratulations by the way!