The bad debt revelation late on Tuesday night triggered the collapse of a $1.65billion capital raising deal which was unveiled at the same time as the increase in bad debt.
Yesterday, in response to an ASX query, the CBA said it only became aware of the increased bad debt provision on Tuesday.
The CBA said it did not immediately disclose the increase because it was an estimate "made in the context of an uncertain economic climate" and analysts had already factored bigger bad debts into their forecasts.
And it said the information was confidential and therefore did not have to be disclosed.
After receiving the letter yesterday morning the ASX referred the issue to the Australian Securities and Investments Commission, which is now examining the case. It is believed ASIC will use the weekend to decide what, if any, action to take.
ASX spokesman Matthew Gibbs declined to talk about the case in detail, saying: "We don't comment on specifics but we are looking at it closely. As a matter of practice, if we remain unsatisfied with the company's disclosure we will refer the matter to ASIC."
An ASIC spokeswoman also declined to comment.
CBA spokesman Bryan Fitzgerald said the bank believed it had not breached any of the exchange's continuous disclosure rules.
"We're happy to co-operate with the ASX and ASIC," he said.
The CBA's bad week started on Tuesday night when it revealed it had raised $1.65 billion at $27 a share in a sale by Merrill Lynch.
In the announcement, CBA revealed bad debts for 2009 were set to rise from 0.5 per cent of the bank's loan book to 0.6 per cent.
Institutional buyers who had signed up for the deal started calling the bank later that night, angry they had not been told of the increase.
The CBA blamed Merrill Lynch for the debacle and the next morning organised a new capital raising at a cut price of $26 a share through UBS.
Merrill Lynch has rejected the CBA's version of events and the dispute appears set to go to court.
CBA scrip rose 4.9 per cent, or $1.30, to close the day at $27.80.
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