Legal provisions for mandatory referendums at national level

Niue
Answer
Yes
Source
Constitutionof Niue
(adoptedin 1974, amended in 1992)
http://www.paclii.org/nu/legis/consol_act/con231/
Constitutional Polls Act 1977 (Adopted in 1977)
http://www.paclii.org/nu/legis/consol_act/cpa1977268/
Comment
Article 35 of the Niue constitution provides formandatory referendum.