Two people have been badly burned after a Christchurch fire.

St John spokesman Gerard Campbell says they've transported two men to Christchurch hospital, one of them in critical condition and one in moderate condition.

The fire started at a block of council flats on Innes Roads, Mairehau.

A NZ Fire spokeswoman says they were called to a flat fire on Mairehau's Innes Road around 3.

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She says two crews responded.

One witness told Fairfax that they heard a 'big bang' and screaming that could be heard through the neighbourhood.