The explosion happened after water was accidentally introduced to molten steel inside the plant’s electric arc furnace.

Australian Workers Union branch secretary Richard Downie said two of the burnt men had been released from hospital but the third was in the Royal North Shore Hospital burns unit. The families involved were distraught, Mr Downie said.

Yesterday’s accident happened at about 12.33am, with police and ambulance personnel attending.

Ambulance officers said a 42-year-old man had burns to ‘‘multiple regions’’, a 40-year-old had burns to both hands and a 37-year-old had ‘‘shrapnel burns to his torso’’.

Mr Downie said another man was knocked backwards down a set of stairs by the force.

A crane driver sitting overhead suffered from the blast.

‘‘It was a lucky escape for everyone,’’ Mr Downie said.

He said the accident happened when a furnace crew was removing a partly melted ‘‘skull’’ of scrap from the inside of the plant’s electric arc furnace.

‘‘The skull somehow dislodged a water-fed cooling plate, which fell into the ‘heel’, which is the molten steel at the bottom of the furnace after it has been tapped,’’ Mr Downie said.

‘‘Because of the water, they thought that if anything was going to happen it would have happened straight away but they waited 10 or 20minutes before they went to get the plate out. But somehow it still exploded.’’

With public concern over industrial safety heightened by events at Orica’s Kooragang Island plant, Mr Downie was confident in Waratah’s safety record given the ‘‘inherent danger of molten steel at 1600degrees celsius’’.

He had been assured of a ‘‘transparent’’ investigation by Waratah plant manager John Barbagello.

WorkCover is also investigating the accident, which follows a molten metal explosion that burnt a worker’s legs on February 15, 2009.

WorkCover said it found ‘‘appropriate safety management systems were in place’’ at the time.