Commonwealth Bank customers were blocked from making online payments and accessing the CommBank phone app for hours on Monday, while non-CBA Visa card transactions at the bank's terminals were down across the country until the afternoon.

The bank issued a statement about 7am saying it was aware of an issue affecting Visa card transactions at CBA merchant terminals and some payments via its online banking service NetBank and its mobile phone CommBank app.

Can't: Commonwealth Bank customers are unable to access their accounts online. Credit:AAP

An unnamed "issue" at one of the bank's two data centres has now been blamed for the problem.

That issue "led to all NetBank and CommBank app traffic routed through our other data centre," a spokesman said on Monday evening.