Drone footage shows the extent of the damage to the Christchurch wastewater treatment plant after a large fire earlier this week.
The fire broke out at the site in Bromley just before 3.10pm on Monday.
Thick black smoke was visible around the city and the Canterbury DHB issued a public health warning.
The footage, posted by the Christchurch City Council, shows extensive damage to the roof of the plant.
Following the fire, operations manager Adam Twose said contractors had been working on the roof when the fire broke out.
"That will form part of the investigation into the cause."
"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."
A contractor on-site suffered a minor superficial injury during the incident.
On Thursday, council head of three waters and waste Helen Beaumont said while it is obvious that the roof and the wastewater distribution system for the trickling filters have been destroyed, it is not yet clear how much of the plastic media housed within the filters has been damaged.
It is also not yet known whether the concrete housing has been structurally compromised, although external inspections indicate there is no immediate risk of collapse.
"To undertake a full internal inspection we will need to gain access to the interior of the trickling filters and remove most, if not all, of the material inside. This is going to be a challenging logistical exercise," Beaumont said.
"There are still hot spots deeper within the filter media – it is about six metres deep. We are assessing options for gaining access so we can do a full inspection once the material has cooled sufficiently for us to safely enter."
Preliminary testing of the debris from the fire indicates that none of the material contains asbestos.
Canterbury medical officer of health Dr Cheryl Brunton has advised that the strong smell created by the fire may persist for some time.
"Anyone experiencing any persistent health issues from the fire should contact their general practice team for advice, in the first instance," Brunton said.
The council is working to install a sprinkler system over the smouldering trickling filters at the plant to help extinguish hot spots and reduce the smell.
"We are aware there are some odour issues around the plant because the plastic media housed within two trickling filters has partially melted and is giving off an acrid smell,'' Beaumont said
"We hope to reduce the smell by putting a constant stream of water through the trickling filters so we are working through options for installing sprinklers over the structures."
While people can still use their toilets, bathrooms and kitchens as usual, the loss of the trickling filters at the wastewater plant is impacting on the wastewater treatment process.
"We are modifying the treatment process so that we can bypass the filters, however, doing that will impact on the quality of the effluent discharge. We have assured Environment Canterbury that we will make every effort to comply with the conditions of our discharge consents and that we will liaise with their compliance monitoring officers during the recovery effort."
Beaumont said it takes about 20 days for the wastewater that is piped into the plant to pass through the treatment process and to get discharged via the ocean outfall so it will be some time before any changes in the quality of the discharge becomes evident.
Motorists are being warned to expect hazardous driving conditions.