Key made his money as a currency trader and owns several properties around New Zealand and the world.
Last month he sold his Parnell mansion for $20 million, adding to his personal fortune.
Key is the sole director of Thirty Eight JK but owns just one share of the company.
Another share is owned by his wife Bronagh Key and the remaining 98 are owned by Key's associate Ken Whitney and another man, Ian Nugent.
Whitney had been described by Key as his lawyer, however Whitney said he had given up his practising certificate in May last year.
Key's been busy since handing the reins of the National Party over to Bill English late last year.
As well as being appointed chairman of ANZ bank's New Zealand arm, he was appointed as a Knight of the New Zealand Order of Merit in August.
He received the honour for services to the state at an investiture ceremony at Auckland's Government House.