Right! Now let's get this straight. My Oxford dictionary rather fudges the issue and so does the online Oxford dictionary. See their entry here.
The word "referendum" is actually two different words with two different meanings:
1) The referring of a matter to the whole electorate.
- This is the meaning which most people will recognize. In this sense the word is derived from a Latin gerund. Now in Latin the gerund is a verbal form which behaves rather like a neuter noun, but as a noun it is defective. It lacks several forms and, in particular, has no plural form. Therefore, if it is used in English as a fully fledged noun, the plural must be formed according to the normal pattern of English, by the adding of -s.
2) A matter which needs to be referred by someone to someone else.
- It may, for example, be used of a matter which needs to be referred by a diplomat stationed abroad to their government at home.
- In this more specialized meaning "referendum" is derived from a Latin gerundive. This is a verbal form which is essentially passive in nature, implies a degree of necessity or obligation and behaves rather like an adjective. It possesses all the adjectival forms and, as with Latin adjectives in general, the neuter form is capable of being treated as a noun. So if it is used in English as a noun, the neuter plural form in -a is available.
Given that the second meaning is rare and that in English it is generally acceptable to pluralize Latin nouns according to the normal pattern of English, I recommend that we always use the form "referendums".