"It was working okay but unfortunately when he opened an inspection cap he was a little bit too close and his face got burnt by the flames inside."
He said the man was in a lot of pain but was able to speak.
When emergency services arrived he was applying cold water to his face from a shower, Mr Williams said.
Greenmeadows School Board of Trustees chair Ross Barr said the school was not going to comment on specifics of the incident.
"There was an incident at the school this afternoon involving the boiler and a staff member was taken to hospital for medical assessment," he said.
"He has received good medical care and out of respect for him and his privacy we are unable to give any more information."
Mr Barr said the school's health and safety team would conduct an investigation.
The boiler would not be used until the investigation was completed.
In 2009, Orewa College caretaker Richard Nel died after receiving burns to 90 per cent of his body when a boiler exploded at the school.
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