Part of Auckland International Airport was closed this afternoon when a passenger declared a bomb in his luggage at Customs.

It is believed the bomb is an unexploded World War II artillery round the man found while snorkelling overseas.

Police are unsure whether or not the ordnance is still live and the bomb squad have been called to take it away.

Police northern communications shift manager Earl McIntosh said the weapon was potentially dangerous to a number of passengers in the airport's International terminal.

"It's unknown if it's live. It's possibly a live artillery round that was brought in by a passenger in his luggage.

"Supposedly the guy had found it while diving. The bomb squad were called from the military to take it away. It may be live, it may not be."

Mike Garland, who had just collected his luggage after flying in from Singapore, said he was "herded" back to the luggage carousel away from Customs by police.

He and the other passengers in the terminal were held there while authorities dealt with the problem, he said.

"The loud speaker said it was a critical incident, or something like that. But no-one knew what it was at the time."

He said the authorities had not divulged what had happened, but a colleague of his had overheard the man declare what he had called "a grenade".

"Suddenly there was a major emergency where everyone was herded away from Customs back toward baggage claim.

"The guy [had] said, 'Some idiot brought a bloody grenade through.'

"I just think it's bizarre. The idiots I think have to held to account."

A Customs spokeswoman confirmed the Customs and Immigration area at the airport had been evacuated after the man declared the bomb.

"My understanding is he was returning back from Tahiti.

"They [Customs staff] obviously called the bomb squad and alerted the airport police when they discovered it."