A truck carrying liquefied petroleum gas (LPG) got stuck under a bridge in Boksburg on Saturday, causing a leak and explosion that initially killed 10 people and badly damaged a nearby hospital.
Eight more people subsequently died from severe burns and other injuries, the health authorities reported on Monday, and eight more died afterwards.
"In terms of numbers, the police has now confirmed that we are standing at 26. So we have lost 26 people out of the tragedy," the prime minister of Gauteng province, Panyaza Lesufi, said during a visit to comfort families of the victims.
Videos on social media showed a huge fireball emerging from under the bridge, which the tanker appeared to have been too high to go under.
The explosion caused "extensive damage to private and public infrastructure within an estimated 500 meters from the scene", the local health department added.
It was carrying 60,000 liters of LPG, which is widely used for cooking.