A passenger from Vietnam arrived in New Zealand with live Siamese fighting fish.

The five fish in plastic bags were seized at Auckland Airport by biosecurity staff after the man declared "wet fish" on arrival forms.

"The traveller's English wasn't very fluent and we initially thought he was carrying frozen or dried fish," said David Sims, Ministry of primary Industries manager, Auckland Airport.

"It was a surprise when he pulled out the plastic bags."

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The fish were "humanely euthanised", MPI said.

"We're pleased the passenger knew to declare the fish," Mr Sims said.

"They came from an unknown source so may have been carrying diseases that could devastate local freshwater fish populations.

"It appears the message is getting through to international air passengers that they need to declare any biosecurity risk items when they arrive in New Zealand."

Mr Sims said the interception was very unusual, although it was not the first time MPI staff had seized live fish at the airport.

In 2013, MPI prosecuted an air passenger after he was caught smuggling live tropical fish in his trouser pockets.

Siamese fighting fish are popular aquarium fish and are native to Mekong basin of Laos, Cambodia, Vietnam and Thailand.