I have tried this in the past with Mixxed results and am still not able to answer a couple of questions. It is suppose to be useless when wet. But have read that it cAn be used as a wormer for animals. suppose to be like a ton of razor blades and it's slices up the insects and they die from dehydration. Seems to work ok in the garden but have always had to use some other type of insecticide to really keep the area bug free. It does make the best water filtration media on earth.
Diatomaceous earth is amazing i suggest buying it if you can - I am unsure how to buy it in New Zealand but I have looked in the past. Diatomaceous earth is so tiny it chokes microbes to death - it also can purify water.
BUT I WOULD NOT USE IT FOR PLANTS - use charcoal, or citric acid with PT water to kill fungicides and insects - even tobacco with water will kill the nasties.