A 56-year-old naturalised New Zealander is in custody in South Korea after he was secretly filmed allegedly handing secret military information to North Korea.



The man is only named as "Kim" by Seoul authorities who say he has business connections in New Zealand.



He is accused of handing details on military equipment and other related information to the North.



Allegedly included was intelligence on a radar navigation system and long-range rocket detectors which Korean media say could have seriously compromised South Korea's military.



A Ministry of Foreign Affairs spokeswoman said the New Zealand Embassy in Seoul was offering consular support to the man.



South Korea's national Yonhap news agency said Kim and another man, Lee, 74, were arrested in May after an alleged meeting with a North Korean agent in Dandong, a Chinese city along the North Korean border, in July last year.



The newspaper Chosun Ibo, quoting the Seoul Metropolitan Police Agency, said Lee was a businessman who had been a prisoner of war from North Korea.



Lee and Kim, who were engaged in trade activities in Nampo, North Korea, and New Zealand, were suspected of attempting to hand over the information at the direction of a North Korean agent.



Police told Chosun Ibo they had footage of their meeting with the agent and a statement from Kim saying he received an order from the North.



The English language Korea Times said a former weapons producer in the South was being questioned over what he allegedly gave Lee and Kim.



"We believe that the suspects engaged in spying activities for North Korea. We've already obtained video clips that show Lee meeting the alleged agent," a police investigator told the paper.



It said Lee was sentenced to life in prison for spying for North Korea in 1972. He was released on parole in 1990, but still maintained his allegiance to the communist regime.



He ran a business in Dandong where he first allegedly received secret orders in July 2011 from a man believed to be dispatched from the North.



The alleged agent was suspected of instructing Lee to collect South Korean military intelligence, including information on high-tech equipment.



Lee then allegedly contacted Kim who had an ex-arms producer acquaintance in the South.



The intelligence they allegedly collected "were manuals that outline details about the equipment, and that is information the public can never have access to," the investigator said.



"If the North Korean agency really has them, it means our military is fully exposed to the enemy."



The Korea Times said the prosecution would also look into any relation between the suspects and North Korea's jamming of GPS signals between April 28 and May 13.



More than 600 flights including South Korean and foreign airlines were disrupted by the jamming.



Espionage can carry a maximum penalty of death in the South, although no one has been executed for any crime since 1997.



The two Koreas remain at war since the North invaded the South in 1950. A tense truce was maintained along the 38th Parallel.



In New Zealand there are around 30,000 Kiwis of Korean ethnicity.