A mystery foreign group won approval to buy a big chunk of New Zealand in a secret $1.3 billion deal last year.

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The sale did not proceed but the deal inflated figures for approved foreign investment in New Zealand to almost $1.9 billion last year, almost five times more than the previous year.

A group opposed to foreign ownership, Cafca, believes the deal was approved in June. But all details, including the foreign buyer's name and the seller's name, were kept confidential, as well as the price. The sale could still go ahead in future.

Overseas Investment Office figures made public yesterday show that in the 11 months ended November, 132 deals were approved, worth a net investment of $1.87b. That was up from $404 million in the same period in 2010.

Big deals already made public include the sales of alcopops group Independent Liquor and soft-drinks company Charlie's to Japanese brewing giant Asahi.

The biggest land deal in the past year was the sale of more than 22,000 hectares of leasehold land at Queenstown's Coronet Peak Station, understood to be to pop singer Shania Twain's former partner "Mutt" Lange.

The mystery $1.3b deal was approved by the Overseas Investment Office, but did not go ahead, research and support officer Peter Hill said.

The OIO kept details secret to avoid "unreasonably prejudicing the commercial position of the parties who are the subject of the information".

Buyers and sellers can ask for information to be kept secret, but the OIO makes the call on that.

A deal of such size would require an asset of the scale of the $1.2-billion-a-year aluminium smelter at Bluff.

Cafca said the decision where details were kept secret was "not unique".

Council of Trade Unions economist Bill Rosenberg, who was formerly involved with Cafca, calculated the deal was worth just over $1.3b, after excluding all other deals for that month.

He also calculated it included 3586 hectares of freehold land and 24ha of leasehold land, and involves the transfer of a half share to overseas owners.

Campaign Against Foreign Control of Aotearoa organiser Murray Horton said they had been unable to find out what the transaction was.

Often when foreign buyers were willing to pay top dollar for assets and land in New Zealand, locals could not compete, he said.

"Farmers are getting priced off the market" with large international corporations buying up farms.

While 132 purchases by foreign buyers have been approved in the past 11 months, the most controversial deal, for the Crafar farms, remains undecided. There had been an expectation that the deal to sell to a Chinese company would be approved after the election because of its sensitive nature. But it was not included in the decisions for November.

The Shanghai Pengxin bid for 16 Crafar farms, understood to be for $220 million, has been waiting nine months for an answer from the OIO on its application to buy the central and southern North Island Crafar farms.

A bid in 2010 by Hong Kong's Natural Dairy was declined because it failed the "good character test".

Cafca believes there are "ructions" at the highest level in the National Party and a split about selling the farms to an overseas buyer.

There are no official figures on how much land foreigners own in New Zealand, but Cafca has estimated in the past that it is about 7 per cent of productive land and that figure is probably higher now.