First National Bank's online and electronic banking services were unavailable for more than an hour on Tuesday after a power failure in Randburg.
The institution's chief executive officer, Michael Jordaan, said on his Twitter account the problem was caused when the back-up power supply failed to kick in.
By 6.30pm the backup system was running smoothly, he said.
Raj Makanjee, the bank's chief information officer, said the error occurred around 5pm. - Sapa