Mark, welcome to the forum!
I fish with a kite dropper rig and I suppose they are a little different in the way they work. I fish on the west coast of Auckland (Muriwai), Kaipara Harbour, Ninety mile beach etc where the current can be pretty strong.
I initially used the sinkers that came with it and they were CR#P!! I had the line tangle up real bad numerous times. When I purchased the kite the guys in the shop encouraged me to watch the Pauls Kite Fishing VIDEO beofre making the purchase which had real life situations setting the longline and retrieving types of catch etc.
They immediately said use 16 - 20oz sinkers if fishing on the west coast or in strong currents, I initially tried tying a few 6oz sinkers together and still had problems until I purchased the big ones and then never had a problem since. So depending on your situation with current I am sure you can run a heavier sinker to ensure your line stays on the bottom.
If the sinkers are too light you could potentially be pulling the whole line off the ground, I am not sure how much lb's tow the torpedo has but not enough weight would be no good I would assume. Try the guys at Pauls kite fishing, they speciallise in all kinds of long lines and have a very good reputation.
The website also has hot spots for kite fishing in NZ, so you may need to get in the right spot. It also took me atleast a year or so to finally dial into the spots to fish, the times to fish, and what are the best weather conditions so it may take a while if you are a first time owner too.
It may also be your bait presentation, maybe try some Soft plastics on the hooks because you basically troll these long line for 3km's so if it doesn't go past a fish in that distance then you need to move.
Good luck and keep us posted!
The Fish
Think like the fish, breath like the fish. I AM THE FISH!