A $450 million class action is being launched on behalf of National Australia Bank shareholders who lost money during the global financial crisis because of NAB's exposure to toxic debt.

Legal firm Maurice Blackburn will lodge the claim in a Victorian court tomorrow.

The firm says NAB had bought $1.2 billion in collateralised debt obligations (CDO) in 2006 which had a heavy exposure to the US sub-prime housing market.

It will allege that between early January and late July that year, NAB failed to properly disclose to shareholders all material information relating to its CDO exposure.