ubergeeknz:keewee01: Sick Leave is not a right, it is a privilege
No, it is a right in this country.
http://www.dol.govt.nz/infozone/businessessentials/basics/leave/sick-leave.asp
I don't think Air New Zealand have acted within the law. Unless they have some kind of evidence she was not taking it appropriately, to care for her sister, they should not be allowed to do this.
Section 68 Holidays act does not specify the form in which proof of sickness should take, it does say "may include a certificate from a medical practitioner that—" therefore other proof is not excluded However it does state this under 68(4) (b) "subsection (1) or subsection (1A) does not give the employer the right to require the employee to obtain the proof from a person specified by the employer.", that does make AirNZ request cloudy, but is it the Authority making the request or AirNZ?