Localisers have sidelobes. There's usually a reverse-sense null just outside of the +/- 35 degrees azimuth service volume.
False glide slopes are pretty much part of the design and they are there. However, the first false glide slope is likely to be found at about twice the height of the normal glide slope and, in addition, reverse-sensing. If you manage to fly down a reverse-sensing glide slope at twice the nominal angle and not notice that something is amiss, it's impressive! Flying down a normal-sensing false glide slope will be a crash dive, as the first such null is found at about three times the nominal angle.
Still, that's good reason to always catch the GP from below, to do your altitude checks and be conscious of the descent rate when descending on the GP.
The Air NZ flight was the victim of something else entirely, as part of the signal normally forming the GP was turned off for maintenance purposes.