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Deputy Chief Fire Officer Trevor Tilyard said staff were testing fire hoses about 2pm, when one burst.
Because of the high pressure, the hose began flailing about and a coupling struck Senior Station Officer Lindsay Rae on the leg.
Mr Tilyard said it was a rare event.
Mr Rae was taken to Dunedin Hospital with a suspected broken leg.
Late last night, a hospital spokeswoman said he was in a stable condition, and had been moved to an orthopaedic ward.
Depending on the extent of his injuries, the Ministry of Business, Innovation and Employment might also launch an investigation into the incident, Mr Tilyard said.