Issues in relation to which referendums are mandatory at the national level

Constitution 2006

http://www.wipo.int/wipolex/en/text.jsp?file_id=195746

86. Amendments requiring support of referendums A bill for an amendment of a provision of the Constitution regarding the status of Bislama, English and French, the electoral system, or the parliamentary system, passed by Parliament under Article 85, shall not come into effect unless it has been supported in a national referendum.

Not all constitutional amendments require mandatory referendum. See source for details.