A man who arrived at Auckland Airport last weekend with live fish in plastic bags said they were for his son's fish tank.
The Vietnamese traveller declared to biosecurity staff that he had Siamese fighting fish, a type of goldfish.
Ministry for Primary Industries spokesperson David Sim said initially staff thought he was carrying frozen or dried fish.
"He said 'I've got some fish', and they asked what type of fish - dried or frozen - and his reply was that they were 'wet fish'. They were in the bag, in water. So we sent him down to the inspection lane and yes, consequently there were five different bags of fish."
The freshwater fish were seized and euthanised.
Mr Sims said the fish could be bought in New Zealand but it was illegal to bring them in because they could be carrying diseases.
In 2013, a passenger was prosecuted after being caught smuggling live tropical fish in his trouser pockets.