Oei Hong Leong, who’s suing Citigroup Inc. after alleging he incurred trading losses of more than $1 billion, said the bank’s information was “critical” to his investment decisions.

Oei was responding to Citigroup’s private-banking arm, which said in a Singapore High Court filing last month it didn’t give the millionaire inaccurate or misleading information. The New York-based bank said Oei knew the “considerable risks” of his investments and made his own decisions.

The businessman, who sued Citigroup in May, said however “sophisticated and experienced” he was as an investor, the bank’s information about his margin position was of “critical importance” because he used it to gauge how much risk to take on, according to his reply filed with the High Court this week.

Oei’s claims are “contrived afterthoughts,” and are unsubstantiated, Citigroup’s filing last month said.

The local press reported in May that Oei lost about $1 billion, which he has repaid in full. He is asking the court to assess damages. Oei was ranked 27th in the list compiled by Forbes magazine of Singapore’s 40 richest people in 2008, with an estimated fortune of US$210 million ($304 million).

Citigroup said in an e-mailed statement today it will defend its position “vigorously,” declining to comment further because court proceedings are ongoing. Oei couldn’t be reached for a comment at his office.