According to local media, the tanker, which was carrying liquid petroleum gas, must have scraped the roof of a bridge near the Tambo Memorial Hospital.
According to the rescue service, the friction between the truck and the underside of the bridge must have caused the first explosion.
When firefighters began the rescue operation, a second large explosion occurred that destroyed a fire engine and two other vehicles, the BBC reports.
At least eight dead and 50 injured
At least eight people died in the explosion and around 50 were injured.
According to local newspaper The Citizen, pictures and videos shared on social media show bodies lying on the ground and badly burned awaiting medical help.
- Going through this was a sight of absolute destruction.
I have never seen anything like this ever before, says local resident Simon Lapping to the newspaper and states that he has seen the bodies of six adults and two children.
Eyewitness Michael Kulinji, who spoke to the Reuters news agency, describes the explosion as "more like a bomb", and according to witness Jean Marie Booysen, the explosion felt like "6.5 on the Richter scale".
People fled the scene
Videos posted on social media show the dramatic explosion with people fleeing the flames and vehicles driving away from the scene.
According to AP, several houses were damaged in the explosion, and patients from some wards at Tambo Memorial Hospital were forced to evacuate.
- The disaster management group is on the ground helping the victims of this tragedy and evaluating the cause of the accident, says Tania Campbell who is the mayor of the municipality of Ekurhuleni where Boksburg is located, AP reports.
She emphasizes that those responsible for the explosion will be held accountable.