shark, I believe NZTCF is symbol for one share of common stock traded over the counter in US.
NZT is the ADR.
There is very little, almost nonexistent, volume in NZTCF. The spreads are a mile wide OTC, so that is probably the main reason why they don't track exactly. Also the mm otc is taking currency risk US Dollars vs Kiwi dollars.

Just my opinions.

Good Luck