Is the threat to release Foot & Mouth disease on Waiheke Island terrorism? The Terrorism Suppression Act 2002 legally defines a terrorist act" as an act intended to cause certain specified outcomes, carried out in order to advance a religious, political or ideological agenda, and with the specific purpose of inducing terror in the population or compelling action or inaction from the government or an international organisation. But while "introduction or release of a disease-bearing organism, if likely to devastate the national economy of a country" is one of those specified outcomes, and demands (relating to the payment of money and changes to the tax system) have been made (meaning an intent to compel), political or ideological purpose may be lacking. It really depends on whether the demands are being made because the authors of the demands are ACT voters disgruntled at their party's poor showing in the polls - or whether they're just greedy arseholes.
The second point to note is that it's only terrorism (or rather, conspiracy to commit terrorism) if the threat is real. If it is simply a hoax, an attempt to extort money with an empty threat, then it is technically "threats of harm to people or property" instead. This carries a penalty of seven, rather than yen, years in jail - but that's probably sufficient.
Either way, I'm hoping that the people responsible are caught and subjected to the full penalty of law.