Importing a prescription medication from overseas is very similar to trying to get any illegal drug (marijuana, P, etc) shipped into NZ according to "the man". I strongly recommend going through your local vet before attempting this.
As for wormers, I've used Drontal many times with no ill effects. The thing is, you treat a 20L bucket and "dip" the fish, not the entire tank, and you only do this when the fish refuse to eat. Otherwise, you medicate discus tucker (1/4 pill per 3 blocks of tucker) and feed it to the fish right away as meds have limited lifespans when mixed with food.
Sorry for your loss and if I may speak from experience; if you make a mistake with meds, don't pussyfoot the water changes. If I medicate a tank and notice ill effects it's an immediate 50% water change on the entire tank. While running around with buckets of pre-warmed dechlorinated water for a couple hours isn't very fun, it's rewarding to see the fish feeling better and can save heap$$$.