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Firefighters were called to the blaze at the Bromley plant just before 3.10pm.
Thick black smoke was visible around the city and the Canterbury DHB issued a public health warning.
Fire and Emergency NZ spokesman Andrew Norris said firefighters had contained the fire, but it was still not out.
Norris said fire staff will be meeting with council staff this morning and working with Environment Canterbury on air monitoring.
The plant is operating at a limited capacity.
"The roof caught fire and that collapsed in and has burnt through the plastic media so we have four appliances still here at the moment to try and get the flames out."
Twose said a contractor on site suffered a minor superficial injury during the incident.
"That will form part of the investigation into the cause."
Christchurch residents can still continue to use water and flush the toilet as normal, he said.
"My staff have started turning up to site this morning. We still need to wait for clearance from FENZ before we can get on to the rest of the site and work through that.
"I'll be getting engineers and project managers coming out from civic to start working out what the process is going forward from here."