Homeopathic products are made by diluting a substance that causes similar symptoms to the condition they're meant to treat. In the case of Hyland's Teething Tablets, that substance is belladonna, also known as deadly nightshade. Wikipedia says that it's "one of the most toxic plants found in the Eastern Hemisphere".
The concern with this particular product is that the serial dilution process is not consistent, and that some tablets contain more deadly nightshade than they are meant to.
In the US it is suspected that the deaths of 10 babies were caused by the product, manufactured by the "Standard Homeopathic Company".
Janet Woodcock from the FDA said:
"The body's response to belladonna in children under two years of age is unpredictable and puts them at unnecessary risk"
The NZ Herald says that "It appears the products are not available on shelves in New Zealand". However, I found an online shop in NZ which is selling these tablets. There is one shop which is currently out of stock. And one which, thankfully, has stopped selling the tablets.