The explosion at 7:40 am tore through the 5-milimeter thick lid of the 15-meter high tank, which can store 12,500 litres of liquid, in the Nuoc Man storage depot, Da Nang City.
A fire broke out and lasted for about five minutes.
The tank was in the final stages of construction along with four others and the constructor, Petrolimex Construction Company No. 3, had filled the tank with water to test it.
A technician was quoted as saying that some gasoline had been mistakenly pumped inside the tank along with water and welding sparks ignited the explosion.
An investigation by authorities is now underway.