A whistleblower at Nissan told a Tokyo court on Thursday he had been instructed to find ways of boosting the retirement benefits of the fugitive former chairman Carlos Ghosn.

Nissan executive Hari Nada was testifying for the first time at the trial of Greg Kelly, who was a top aide of Ghosn when they were both arrested in 2018.

Nada was a key player in Ghosn's sudden arrest, which sent shockwaves through the business world. He obtained whistleblower status from prosecutors for his cooperation.

The Japanese auto giant's disgraced ex-boss Ghosn is at large after jumping bail and fleeing Japan for Lebanon in 2019 -- leaving Kelly as the only person facing trial in the rollercoaster saga.

Kelly, 64, denies charges he conspired to under-report tens of millions of dollars in pay that Ghosn was allegedly promised after his retirement.

Nada told the Tokyo District Court he was instructed by Kelly to find ways to increase Ghosn's retirement payment.

He said he had found that Ghosn's retirement scheme did not include stock appreciation rights, or SARs.

Former Nissan chairman Carlos Ghosn was arrested in 2018 for alleged financial misconduct
Former Nissan chairman Carlos Ghosn was arrested in 2018 for alleged financial misconduct AFP / ANWAR AMRO

"The conclusion is we could include (SARs) into the calculation," Nada said as he stood to give testimony, dressed in a dark suit.

Asked how much SARs could raise Ghosn's retirement payment by, he said: "About 2.4 billion yen ($23 million)... something like that."

As Nada spoke, Kelly sat next to his lawyers, taking notes quietly.

Nada said Ghosn thought he would lose his job if his high salary was disclosed, as the French government would not look favourably upon it, public broadcaster NHK reported.

He also said he was instructed by Kelly to secretly consider a merger between Renault and Nissan, according to NHK.

Kelly's trial began in September and is expected to last until the summer.

It centres around the question of whether Kelly and Nissan between 2010 and 2018 illegally concealed payments of around 9.2 billion yen ($88 million at today's rates) promised to Ghosn on retirement.

Former Nissan CEO Hiroto Saikawa will also testify in the trial next month.