The situation really is that you need to present a STATEMENT OF SERVICE as opposed to an employment certificate. It is fine to have worked as a volunteer but you in fact worked as a Registered Nurse on a voluntary basis. Australia does not recognize the term volunteer nurse. The statement of service must state that you worked there from this date to that date a total of x hours as a Registered Nurse and these were the duties that you performed. As long as the duties performed are the duties of a Registered Nurse then that is OK. Hope that I have been of some help to you all.