Whether this is a privacy issue depends very much on the subject matter and the context. In this respect it comes down to legitimate expectations of privacy in the case of adults and appropriate consent when it comes to children (at least in terms of the Protection of Personal Information Act). The law that is likely to be more appropriate here is the Protection from Harassment Act which targets forms of harassment which the Act defines as follows:
"harassment" means directly or indirectly engaging in conduct that the 5 respondent knows or ought to know-
(a) causes harm or inspires the reasonable belief that harm may be caused to the complainant or a related person by unreasonably-
(i) following, watching, pursuing or accosting of the complainant or a related person, or loitering outside of or near the building or place where the complainant or a related person resides, works, carries on business, studies or happens to be;
(ii) engaging in verbal, electronic or any other communication aimed at the complainant or a re.lated person, by any means, whether or not conversation ensues; or
(iii) sending, delivering or causing the delivery of letters, telegrams, packages, facsimiles, electronic mail or other objects to the complainant or a related person or leaving them where they will be found by, given to. or brought to the attention of, the complainant or a related person; or
(b) amounts to sexual harassment of the complainant or a related person;
The harm the Act protects against may be "any mental, psychological, physical or economic harm".
This Act is designed to be user friendly and the Regulations describe which forms to use for which steps and who to approach at each step. The Department of Justice and Constitutional Development has a comprehensive page with links to the Act, the Regulations and the various forms. The process was designed in such a way that you don't need an attorney to assist you (although you can have one helping you) and you need not know the harasser's identity either. The Act creates a mechanism whereby the police may be instructed to investigate and identify the suspected harasser.
This legislation can be used for a various activities which fall into the "harassment" definition including stalkers like the ones described in the segment as well as cyber-bullying and more.