A spark igniting acetylene gas leaking from a welding torch that had not been switched off is now suspected as the cause of the fierce explosion which ripped through an engineering and metalwork classroom at Kaitaia College.
Five teenage boys and their teacher were injured in Monday's blast and taken to
hospitals in Kaitaia, Whangarei and Auckland.
Four boys remain in hospital, two each in Whangarei and Auckland, where all have now been transferred from intensive care units to general wards.
Their condition in each case was described as stable yesterday.
Teacher Kevin Subritzky has been discharged from hospital, while a fifth boy was treated and discharged on Monday.
Far North fire safety officer Lindsay Murray said yesterday investigations were at an early stage but it appeared that acetylene gas had leaked from a welding torch that had been left on.
The highly flammable and colourless gas, used in welding and metal cutting, had built up.
A spark would have ignited the gas but exactly what caused the sparked ignition was still unclear.
Acetylene and oxygen cylinders were stored together in a separate room from the main workshop area and had not exploded, Mr Murray said.
No fire had accompanied the explosion.
The classroom and workshop are closed while Occupational Safety and Health continues its probe.
Eighteen boys, their teacher and a teacher aide were in the room when the blast occurred. Most of those injured received head wounds and lacerations from flying tools, glass and other material.
