I am not certain you are aware just how restricted NZ fish keepers are with what sort of medications are available for fish. There is an extremely limited selection of legally imported remedies here in New Zealand and it is very hard to find a Veterinarian who has sufficient knowledge of aquatic creatures, let alone get them to prescribe something for a fish.
I see you have tested your ammonia levels. Have you checked your pH, nitrates and nitrites. While I don't necessarily think these are the issue it is possible that your fish were carrying something when you purchased them and the stress of being moved may have triggered it. Google this - columnaris disease - and see what you think. Furan 2 is the best non scripted medication available for bacterial infections.