The National Bank of Australia will face a $450 million class action lawsuit for failing to reveal a massive debt dated two years ago.

Brooke Dellavedova, a spokesperson for the lawsuit, said several institutions and retail investors already signed up for the claim against one of Australia's biggest banks.

Ms. Dellavedova said the NAB failed to disclose the bank's exposure to toxic US debt earlier in 2008 and shareholders were left in the dark when shares prices declined later that year.

"What we say is you had these assets, the rest of the market seemed to understand that they were heavily impaired, but you did nothing," she told media reporters.

"In fact you came out and said your assets were fine and then [in] two months took a very serious hit on those assets."

A representative of NAB said the company refused to comment and is yet to receive any communication with regards to the class action.

NAB is also one of the banks facing another class action over dishonor and late fees charged to customers over the past six years.

NAB last week also denied it started refunding some fees to customers after the news of a lawsuit against banks came out.