A spokesperson told ZDNet Australia the bank wasn't sure what was causing the problem or when it started. However, he said it was being investigated.
Customers unable to login to the service -- named NetBank -- are instead greeted with a screen advising them to re-login as "some difficulties are currently being experienced".
The incident is the latest in a string of glitches since NetBank's AU$100 million upgrade in April.
Several weeks ago a bug in the system resulted in duplicated transactions for some customer accounts. A number of other user interface errors have also surfaced.
