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Christchurch City Council head of three waters and waste Helen Beaumont said an engineer is at the Bromley plant assessing the damage.
The fire broke out on Monday, causing thick black smoke to spread across the city and prompting the Canterbury DHB to issue a public health warning.
The blaze was eventually brought under control about 6pm, but the fire remained active until noon the following day.
"A structural engineer is currently on site undertaking a preliminary external investigation of the two trickling filters, where the fire was concentrated," said Beaumont on Wednesday.
"It may be some days before we can do a full assessment of the damage as the trickling filters are still smouldering and will need to have water applied to them for several days yet," said Beaumont.
"However, the early indications from the engineer is that there is no immediate danger of the structures collapsing.
"We appreciate that the public want to know more about the extent of the damage to the plant and we will release information as soon as we have it.
"The wastewater plant is still running and we are progressing well with the optimisation of the treatment processes. We are liaising with Environment Canterbury on how we can adapt the plant while the trickling filters are out of action,’’ Ms Beaumont says.
"At this stage the quality of the discharge from the wastewater treatment plant is still typical of normal operations but there is a lag of about 20 days between wastewater entering the plant and the discharge from the ocean outfall. We will be closely monitoring this over the coming weeks.’’