Westpac's online broking service was offline for most of Thursday, with customers having to use the bank's phone broking service instead.
A Westpac spokesperson said that the outage came as a result of a patch that needed to be removed following a regular upgrade to the system the previous night, and that it "wasn't anything irregular".
"This morning it was necessary to remove one of the patches and that removal affected the public operation of online broking," the spokesperson said.
However, it appears that customers were not informed of the regular upgrade and the resulting outage, but were instead greeted with a message on the Web page this morning.
An early message advised customers who had made trades after 9am to call Westpac. After some time, this part of the message was taken down, with customers merely told that staff were working to get the system back online and advised to use the phone broking service.
The spokesperson said that customers who had placed trades after 9am were required to contact the broking group as a "back up to make sure [trades] were processed".
Although the spokesperson said that customers were placing trades as usual, and that the waiting times for the phone service were no longer than usual, one customer who wished to remain anonymous claims that the inconvenience cost him about $800 on a trade that he wanted to make online earlier in the day.
The spokesperson also added that customers were only charged Internet broking rates on any trades made via the phone system during the outage.
This latest incident follows Westpac's online banking outage last month, with customers unable to access the service for a number of days, reportedly due "loading capacity" problems.