Technician Martin O'Connor died in the Atlas Gardens factory blast on Wednesday, spokesman Wilfred Solomons-Johannes said.

Saji Thekkemalayil Chandrasekharaan lost a leg in the blast and died in hospital overnight.

Two others were recovering in intensive care. Brandon Pretorius had both legs amputated and was in a critical but stable condition. Jean Rossouw was serious but stable.

The city's fire and rescue services spokesman Theo Layne said four others sustained minor injuries.

Solomons-Johannes said a private service provider, Global Gases SA, had been contracted to repair a compressor inside the factory. The gas company happened to be on the same site as the factory.

Around 4pm, a compressor and gas cylinders exploded, ripping out the windows and sections of the corrugated iron roof.

The building was declared structurally unsound.

"The site is still closed off and body parts are lying there. The body remains on scene and there will be gathering of evidence," Solomons-Johannes said.

The cause of the blast was being investigated.

India's High Commissioner had been informed of the blast and would notify Chandrasekharaan's family of his death.