Panamanian law makes the ultimate owner of this stretch of Kiwi coastal paradise "difficult to track".

The Overseas Investment Office has admitted it may have lost track of who ultimately owns the Onetai Station farm in Taranaki.

The 1317 hectare farm was sold in 2014, on the advice of the Overseas Investment Office (OIO), to Ceol & Muir, a company established with the help of Panamanian law firm Mossack Fonseca.

The OIO advised ministers in 2013 that Ceol & Muir was "ultimately owned" by Argentinian businessmen brothers Rafael and Federico Grozovsky.

But OIO group manager Annelies McClure said ownership of Ceol & Muir was determined by who held 13 separate "bearer certificates", three of which it has been unable to trace.

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Each of the certificates, which work similarly to a blank cheque, confers either 5 or 10 per cent ownership in Ceol & Muir on whoever holds that certificate.

"The issue of bearer shares means that it is difficult to track changes of ownership," McClure said.

"The OIO does not know whether the Grozovsky brothers still remain the 100 per cent shareholders of Ceol & Muir.

"We have been advised by the solicitors for the Grozovsky brothers that Panamanian law does not require a public share register to be maintained."

Ceol & Muir's solicitors provided copies of 10 of the certificates on April 29, she said.

But copies of remaining three certificates were not supplied and the OIO was "following this up", she said.

Labour MP David Cunliffe has alleged the Grozovsky brothers were held "criminally responsible" in Argentina for a toxic leak from a tannery that they owned.

But the difficulty the OIO is having establishing the ownership of Ceol & Muir goes closer to the issues raised by New Zealand's foreign trust controversy.

That is whether the laws governing the disclosure of foreign interests in trusts and companies are adequate.

Documents showed the treasurer of Ceol & Muir – Uruguayan resident Gustavo Daniel Chaves Mantaras – also directed a company that Portugal alleged was used by former Brazilian soccer coach Luiz Felipe Scolari to evade tax and launder more than €7 million (NZ$11.8m) of income he received while working in Portugal.

The Portuguese investigation was dropped after Scolari reportedly agreed to pay Portugal's tax authority €3.9m.

McClure reiterated that the Grozovsky brothers had attested to an ownership diagram in 2013 that indicated they had a 100 per cent shareholding in Ceol & Muir.

"The OIO has been told by the solicitors for the Grozovsky brothers, in response to a statutory demand for information dated 22 April 2016, that 'our client believes that the ownership structure was correctly represented to the OIO at the time of the application and at the time of settlement and that there has been no change to bring to the attention of the OIO'," she said.

But McClure said the OIO was looking further into the information it received in 2013, saying there might have been "misunderstandings" because of language and legal issues.

The OIO had sought "further clarity" on a email it received from law firm Kensington Swan which indicated Ceol & Muir was "effectively owned by, and the trustee for, Rafael and Federico Grozovsky family interests", and on a file note that Ceol & Muir was a "blind trust with local directors and shareholders", she said.

The OIO asked Kensington Swan for the split in the shareholding between Rafael and Federico Grozovsky in Ceol & Muir in December 2013, but McClure said it "appears from the OIO's file that the solicitors did not respond to that request".

Despite the non-response, the OIO recommended ministers approve the $6 farm sale, which they did the following February.

The OIO is reviewing the application while Land Information Minister Louise Upston has requested an independent inquiry into the processes the OIO uses to establish buyers' "good character".

A spokesman for Upston said person chosen to carry out the inquiry would not be asked to look at the processes the OIO used to establish buyers' identity, but indicated they could nevertheless decide to include that in their inquiry.