Herbal remedies are very popular these days, with many pharmacies in New Zealand happy to promote products that don't work as treatments for medical conditions, or even just as a preventative measure - a way of keeping healthy.
Apart from homeopathic doses of herbal remedies, which usually contain nothing of the listed ingredient, herbal pills normally contain active ingredients, often at levels that will affect the human body. Although these treatments have not been proven to be efficacious in treating the medical conditions they claim to help with (hence why they're often called "alternative" medicine), that doesn't mean that they don't do anything to the human body at all. Sometimes the side effects of these herbal concoctions can be damaging or even fatal.
This isn't the only time that green tea extract has caused the sudden failure of someone's liver. There's another case in Australia from 2016:
Often these products are sold as foods or health products, rather than as medicines, and don't come under strict enough rules to guard against these kinds of issues. Unfortunately, in New Zealand, our new government recently decided to scrap the Natural Health Products bill, which would have done a good job of ensuring that dangerous doses of unproven natural health products are not sold to the public.
There's a great quote from Jim that sums up the issue quite nicely:
"I didn't expect harm. I expected that I might waste my money, I may take these and they don't do a bit of good. I can accept that risk, but the risk that it could cause my liver to fail, that's a risk that's too high for somebody to take."
That is the case with many herbal supplements and other alternative health products - there are unlikely to be any benefits, and there's a small but real risk of serious harm. As always, the best advice is to talk to your doctor about any health issues or concerns you have - and we're talking about a real medical doctor here, not a Naturopathic "doctor" - and follow their recommendations.