CITIC Securities Co, China's biggest brokerage, overstated some of its financial derivatives by more than 1 trillion yuan ($166 billion) in monthly reports from April to September, the country's securities association said.

The Securities Association of China (SAC) said that CITIC reported inaccurate numbers in statistical reports on over-the-counter equity swap deals partly carried out during the summer's stock market rout.

The SAC, a quasi-regulatory body supervised by the China Securities Regulatory Commission, said it was investigating the matter, but has not accused CITIC Securities of any wrongdoing.

As the wrong figures were not included in CITIC's quarterly financial filings, the brokerage's reporting error has no impact on its reported earnings, an investor relations representative at CITIC told Reuters, and CITIC's shares were largely unmoved by the announcement.

The over-the-counter derivatives figure was simply "a business indicator required by the regulator", the representative said.

In an emailed statement, CITIC said the error was caused by a system upgrade, and the figures were amended at the beginning of this month.

The issue emerged amid wide ranging investigations by the authorities into brokerage misdemeanors after China's stock markets crashed in the summer. The official Xinhua news agency reported in August that four CITIC Securities executives had owned up to insider dealing.

CITIC said the current size of its swap business is no larger than 40 billion yuan.