The City of Joburg has identified three possible causes of the explosion on Lilian Ngoyi Street in the city centre.

City manager Floyd Brink said it identified the ignition of methane gas in underground stormwater systems due to sewerage ingress, the ignition of natural gas mixed with oxygen in underground stormwater drainage systems or service ducts, and the ignition of gas from a gas pipe burst.

Brink addressed the media from Mary Fitzgerald Square in Newtown on the progress of the preliminary investigations and the City's response to the explosion.

One person died and 48 people were injured when the underground explosion ripped parts of Lilian Ngoyi Street, damaging several vehicles, some of which fell into a sinkhole, on Wednesday evening.

When News24 arrived at the scene on Wednesday and Thursday, some residents in surrounding apartment buildings evacuated while others remained.

Those who evacuated said they would return when they were reassured that the buildings were safe, while those who stayed said they had nowhere to go or could not afford to pay for alternative accommodation.

Brink said preliminary assessments showed that buildings were structurally sound and there was no need for residents to evacuate. He said no cracks or broken windows could be attributed to the explosion.

"Buildings adjacent to the collapsed road have been inspected, and at this stage, no signs of failure have been detected. The buildings are regarded as structurally sound for occupation."

Brink added that environmental services and air quality controllers said the gas leak did not compromise the air quality. The experts said the gas concentration becomes weak in the open air and therefore poses no threat.

He said other measures by the City included the deployment of Joburg EMS personnel to monitor the affected site and Joburg Water had temporarily cut off water supply.

"Johannesburg Water has already isolated water supply to the area and has deployed water tankers to support residents. This will also be augmented by the deployment of mobile ablution facilities to provide for residents in the affected area," he said.

He said Egoli Gas was shutting down its gas pipes at the site of the explosion. It would conclude a complete shutdown on Thursday evening.