A recently discovered secret cache of British Foreign Office documents appears to include a lot of information about New Zealand and parts of the South Pacific.

The vast archive of 1.2 million files was kept by the Foreign Office at Hanslope Park, a high-security compound that it shares with MI5 and MI6 in London, the Guardian reported.

The Foreign Office had not answered questions about the papers and historians said it was difficult to be sure of their significance without examining them.

Historic papers about the slave trade were among the papers dating back to 1662 and wre thought to contain information about England's involvement in slavery, while others were created in the 19th century and detailed British attempts to suppress the trade.

Under the Public Records Acts, the slavery papers should have been handed over to the National Archives, the paper reported.

A Foreign Office inventory of the cache suggested some nuggets on New Zealand.

Every country appeared to have a file and the Confidential Print; New Zealand had "annual reports; diplomatic reports; leading personalities" over the period between 1969 and 1998.

No other detail on the file was given.

The cache included documents about one of the South Pacific's biggest scandals, the destruction last century of Banaba or Ocean Island, now part of Kiribati.

New Zealand farmers were the major beneficiary of the phosphate strip-mining which rendered it unliveable.

A New Zealander, Albert Ellis, discovered that like neighbouring Nauru, Banaba's top soils were rich in phosphate.

Ellis bought the mining rights from its people in 1900 for £50 pounds a year for 999 years - a contract written into New Zealand law.

After World War I, Banaba and neighbouring Nauru were placed under the British Phosphate Commission (BPC) and mined into exhaustion after World War I.

Banaba was part of the British colony of the Gilbert and Ellice Islands (now Kiribati and Tuvalu).

New Zealand and Australia each held stakes of a third of BPC, which was much later exposed for double bookkeeping showing the local people and Nauruans had been robbed of royalties.

When the Japanese invaded in World War II, they took most of the Banabans into slavery on Chuuk (then Truk) in Micronesia.

About 160 islanders left on Banaba were massacred by the Japanese, who pushed them off a cliff days after the war had ended.

The British refused to return the survivors to Banaba at the end of the war, placing them instead on a coconut plantation on Rabi in Fiji, where they remain today.

About 5000 Banabans live on Rabi and just 300 on Banaba, which has only a narrow coastal strip that is inhabitable.

The Banabans eventually sued Britain and the case in 1976 was at the time the longest-running High Court case, lasting 221 days.

Known as "Rottan Tito v Waddell and others and the Attorney General" it ended when the court found Britain had no legal obligation but did have a moral debt.

The description of the secret Foreign Office file said it contained papers used in the court case and constitutional proposals.

There was also a Confidential report and Accounts for 1968 from BPC.

Under the Muldoon government New Zealand kept its BPC files secret.

The cachet included confidential reports on Fiji from 1970 to 1997 which would include independence and Sitiveni Rabuka's two 1987 coups.

Perhaps more historically intriguing was a confidential print on the Pacific Islands.

Affairs of the Society Islands; affairs of Sandwich Islands were listed, referring to French Polynesia and Hawaii.

There was discussion of the Navigation Islands; kidnapping and deportation of natives - a reference to Samoa.

The kidnapping may be a reference to Peruvian slave trading in the 19th century which plundered islands near Samoa.

Also in the file was details of the 1874 "annexation of the Fiji Islands" - something London had not wanted to do.

There were also "Samoan claims arbitration papers; affairs of Tonga; the Ocean Island; phosphate industry report; diplomatic reports ... ."