The banking giant is currently conducting a share placement to institutional investors with a price range of between $38 and $43 a share.

This compares with CBA's closing price on Tuesday of $45.15. The bank's shares were placed in a trading halt before the Australian Stock Exchange opened today.

The placement of up to 52.6 million shares, which is being led by Credit Suisse, JP Morgan, Citigroup and CommSec, is expected to raise $2 billion, market sources said.

The bookbuild is expected to be finalised later today.

"The strong geographical and business synergies make BankWest an attractive acquisition for us,'' CBA chief executive Ralph Norris said.

"BankWest provides us with a significant opportunity to further enhance the group's business in the fast-growing Western Australia market,'' he said.

Mr Norris said he remained committed to BankWest's West Australian network and jobs, however the bank's east coast rollout would be placed under review.

He said BankWest would continue to operate as a stand-alone entity.