A probe into Bougainville's President has found Ishmael Toroama accepted an offer from an Indian company to cover the cost of his wife's life-saving kidney surgery before signing a lucrative mining deal.

According to a report released from a non-profit, global investigation network, the transplant for Mr Toroama's wife, Betty, took place in Coimbatore, India, on December 5 last year, just 15 days after the Memorandum of Understanding was signed between Bougainville - the autonomous region of Papua New Guinea - and Lloyds Metals and Energy Ltd.

Coimbatore Kidney Centre's administrator told the Organised Crime and Corruption Reporting Project that Lloyds managing director, Balasubramanian Prabhakara, paid the medical facility an undisclosed amount for the transplant.

The investigation was released on the eve of PNG's parliament debating whether to ratify the 2019 non-binding referendum where nearly 98 per cent of Bougainville residents voted in favour of independence.

It found Mr Prabhakaran made the offer to the Bougainville President three times before Mr Toroama finally accepted.

According to the investigation, Mr Toroama had chosen to bypass an official bidding process to select Lloyds, a publicly-traded company on the National Stock Exchange of India.

The resource-rich Panguna copper and gold mine has laid dormant since 1989 amid militant activity that developed into a civil Bougainville conflict.

The Organised Crime and Corruption Reporting Project said Mr Toroama decided to sign the deal with Lloyds against advice from the nation's majority-owned Bougainville Copper Ltd that Lloyds lacked the technical and financial capacity against rival bidders.

"Bougainville's majority state-owned mining company spent around 10 months on a public search for an international partner, speaking to some of the world's largest and most experienced mining companies," the investigation stated.

The investigation revealed that Lloyds had "no background in major copper and gold extraction".

Mr Toroama was said to have told the Bougainville Copper Limited, the operators of the mine, to cease ongoing talks with rival, China Molybdenum Company, early.

The order comes after Mr Toroama previously was accused of negotiating a "secretive" deal following concerns raised by traditional landowners over a lack of transparency.

Mr Toroama told the Organised Crime and Corruption Reporting Project Mr Prabhakaran offered to fly his wife to India for a life-saving kidney transplant at no cost to him or his family.

"Toroama did not publicly disclose his acceptance of the apparent gift," the investigation found.

According to Mr Toroama, his wife's operation was just a "personal arrangement" he agreed upon with Lloyds managing director.

He told the Organised Crime and Corruption Reporting Project that he only agreed after it was offered "three times".

"Although Toroama said it was his understanding that the cost of the first lady's treatment was handled by the executive, he said it did not influence his decision to choose Lloyds," the investigation reported.