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

See Constitutional and Other Polls Act 1993 section 4:  http://www.paclii.org/ck/legis/num_act/caopa1993345/   

The scope of optional (advisory) polls is unlimited (except for those explained in R.01 which are constitutionally mandatory).

Any issue, except those which must be put to mandatory referendum.