Legal provisions for optional referendums at national level

Cook Islands
Answer
Yes
Source
The Constitutional and Other Polls Act 1993 is at:
http://www.paclii.org/ck/legis/num_act/caopa1993345/
Comment
See Constitutional and Other Polls Act 1993 section4: The scope of optional (advisory) polls isunlimited (except for those explained in R.01 which are constitutionallymandatory).