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Emergency services were called to the fire in Curzon Place about 6.10am.
Caity Leah lives opposite the house and told Newstalk ZB's Chris Lynch she woke up to a fire alarm and land banging and popping noises.
She said another neighbour carried the elderly man out of the burning house and on to the street.
"It was extremely intense to wake up to and I feel for everyone involved."
Leah said the elderly person was very upset.
A St John spokesman said the man was taken to Christchurch Hospital with serious injuries.
The two-storey flat was extensively damaged on the lower level and is now extinguished.
"The building was well involved on arrival," a Fire and Emergency New Zealand spokesman said.
By about 6.30am, firefighters were still working to extinguish the blaze.
Officers were dampening down the fire at about 7.30am.