Legal provisions for mandatory referendums at national level

Tonga
Tonga’s Constitution and electoral laws are consolidated to 1988with subsequent amending legislation listed by category and Act at these links:
http://crownlaw.gov.to/cms/index.php/legislation/current/all-alphabetically.html(Act of Constitution listed first)
http://crownlaw.gov.to/cms/index.php/legislation/current/all-by-category.html(electoral and related laws).
The Act of Constitution of Tonga and its legislation makes noprovision for any form of direct democracy in the electoral/polling sense. (This might be expected in a nation with aUnited Kingdom legal inheritance mixed with a strong tradition of kingly andnoble rule). From 2010 Tonga is movingfrom a quasi-monarch with a directly elected element to something closer to aconstitutional monarchy with responsible Cabinet government.
However, Tonga does maintain an interesting form of compulsory people’s assembly or ‘Fono’,albeit the assemblies are more hierarchical (formal address + deliberation)than decision-making. See the Fonos Act, which provides for a ‘Great Fono’(addressed by national, elected representative), a ‘Nobles Fono’ (gathering ofthose on hereditary land addressed by noble) or ‘Ordinary Fono’ (town assemblyaddressed by officials).
The closest Tonga otherwise comes to DD is section 8 and 75 of itsConstitution, which mention the ability of people to petition the LegislativeAssembly against a bill, or to complain to a member of the Legislative Assemblyasking for the impeachment of a Minister or elected representative. But these are no more than humble petitionsrequesting action or investigation.
In essence, there is nothing to prevent the Legislative Assembly(subject to Kingly veto) from opting to set up ad hoc legislation for an advisoryplebiscite. But we know of no evidenceof that ever occurring, and there is certainly no standing legislation orconstitutional provision to facilitate it.