National Australia Bank's internet and mobile banking services have resumed almost five hours after frustrated customers vented their anger when the services became unavailable on Friday afternoon.

Customers began to notice the outage about 1.30pm on Friday, taking to social media to quiz the bank on how long the services would be down.

"We seem to be having some issues with our Mobile App and Internet Banking," NAB responded.

"The team’s looking into it now and we’ll post an update shortly."