Dipel was the product I couldn't remember the name of, but I can't find it for sale in New Zealand.

I found it on an Aussie Bunning's website: Yates 40g Natures Way Caterpillar Killer Dipel Insecticide but that's not helpful :(

I bought 1kg a couple of years ago, which will last me the rest of my life as long as it doesn't go off - I store it in a cool, dark, dry place. Have only used 20 - 30 grams in the last 3 years.

When I bought it, it was called Bactur Organic from SprayShop. It looks like that has been discontinued and replaced with Genius BT.

This was the most economical way to buy it for me at the time. It has gone up considerably by the looks. If you've got friends who garden, you could share it around to spread the cost - it goes a long way.

I think there is another more generic brand that sells it, but I can't remember what it's called, and they sell very small quantities for the price.

IIRC => Yep, BT produces a protein that blocks the digestive system of the insect/caterpillar, effectively starving it - we don't see any meaningful loss at all. It is not supposed to harm other beneficial insects, and that appears to be true - we don't spray it on bees, but haven't seen any dead bees or reduced bee numbers with its use.