Who can initiate an optional referendum at the national level?

Referendums Act (1999)

http://aceproject.org/regions-en/eisa/ZW/Referendums%20Act%20Chapter%202%2010.pdf

Article 3Whenever the President considers it desirable to ascertain the view of voters on any question or issue, he may by proclamation in the Gazette:(a) declare that a referendum is to be held in order to ascertain the view of voters on that question or issue