Taiwan's defence minister apologised Monday after claims that military police illegally searched a private home sparked an outcry in the now-democratic island, which spent decades under martial law.

Military police late last month found a man surnamed Wei trying to sell online three government documents dating back to 1960-70 which they believed were classified documents.

Posing as buyers, the military police contacted Wei. The officers, who had no search warrant, said they went to Wei's home and "obtained" the documents with his consent.

Wei said he handed over the documents after the military police threatened to prosecute him for concealing them and for leaking state secrets.

The case came to light after Wei's daughter told the story on one of the island's most popular social media websites over the weekend.

"I apologise to all countrypeople for the uneasiness and disturbance caused by the event," minister Kao Kuang-chi told parliament.

Military abuses are especially sensitive in Taiwan, which was under martial law from 1949 until partial democratisation in 1987.

Yang Chia-liang, spokesman for the main opposition Democratic Progressive Party (DPP), accused military police of abuse of power and infringing a citizen's human rights.

Civilian prosecutors are investigating the case.

The military says the documents were related to Chinese communist spies and confessions they made after they turned themselves to Taiwan's government in the 1960s and 1970s.

But on Monday it acknowledged the documents had been declassified.

The Kuomintang (KMT) party, which fled to Taiwan in 1949 after losing a civil war to the communists on the mainland, ruled with an iron fist for decades.

Taiwan became a full democracy after the first direct presidential election in 1996.

The KMT will hand over power to the DPP in May after losing the presidential and parliamentary election in January.