Most suburbs in Johannesburg lost power on Friday afternoon, but the cause of the outage has not yet been determined.

"We have a huge outage in most suburbs in Johannesburg especially the central and northern suburbs including Sandton, Midrand and Alexandra," City Power Johannesburg tweeted on Friday afternoon at 15:10. "This affected at least 20 of our substations with each servicing at least 10 suburbs."

National power utility Eskom earlier said that large parts of Randburg, Midrand and Sandton had lost power. The outages have caused traffic snarl-ups along numerous routes.

Eskom spokesperson Khulu Phasiwe said technicians were on site to determine the cause of the outage.

"No estimated time of restoration at this stage," he said around 15:00.

Eskom said earlier in the day that the power system remains tight, but the fault in Johannesburg does not appear to be related to planned load shedding.