In the New Zealand context section 23 of the Human Rights Act provides that:
It shall be unlawful for any person to use or circulate any form of application for employment or to make any inquiry of or about any applicant for employment which indicates, or could reasonably be understood as indicating, an intention to commit a breach of section 22 of this Act [s 22 provides that it is unlawful to refuse or omit to employee an applicant by reason of the prohibited grounds of discrimination]Most employers are by now well aware they should not ask question about such matters as age, family status, sexual orientation and so on - asking for access to Facebook is essentially a request for such information.
Account might also need to be taken of the privacy principles set out in the Privacy Act. Principle 1 requires that the information be necessary and principle 4 that the means of obtaining the information "must not intrude to an unreasonable extent upon the personal affairs of the individual concerned."
Modern employers seem only to ready to pry into all aspects of the lives of their employees-this might at least inhibit one form of intrusion.