The Commonwealth Bank of Australia today moved to reassure its customers that no hacker had accessed their accounts.
A bank spokesman said claims by a hacker on talkback radio this morning that online accounts could be accessed were being investigated and he reiterated customers' money was safe.
The hacker referred to only as Mr Williams said he could hack into the bank's online service and has details of 50,000 customer accounts.
He said a flaw in the CBA's quick line service allowed him to break into accounts to potentially steal unlimited funds.
A bank spokesperson said the matter is being taken seriously but emphasised its online banking service had not been hacked.
She said the bank would be happy to meet with Mr Williams to discuss the allegations after it had carried out its investigation.
A former IT security director at one of the big four banks told CW the problem is not with the Commonwealth's core systems but is the inherent insecurity of the user's own computers.
He said a hacker can access user details stored on the users' own computer by analysing downloaded code.
"This is not the first time this has happened and similar problems exist with other banks and e-commerce transactional Web sites," the security source said.
"In this instance Mr Williams has capitalised on the media hype to embarrass the Commonwealth Bank; the tragedy is that consumers will focus on the bank's problems without thinking through the real issues involved."