Christchurch City Council is warning of an increased risk of unpleasant smells from its wastewater treatment plant after conditions in two oxidation ponds deteriorated this week.

Ponds 1 and 2A have moved to red on the council’s odour status tracker, indicating a high probability of smells affecting nearby areas. Pond 2B has moved to orange as similar signs of declining health begin to emerge.

However, the council said its community monitoring network had not recorded any significant spikes in recent days.

The decline comes while Clarifier 4 is offline for essential repairs, directing more load to the oxidation ponds and placing additional pressure on the system.

The council said staff were adjusting loading and flows across the ponds to support their health. Of the plant’s 41 aerators, 39 were operating, with two temporarily offline because of an electrical issue.

“We know the potential for odour is a concern for residents, particularly those living near the treatment plant,” the council said.

“Please be assured we are closely tracking conditions and will provide a further update early next week.”

Repair work on Clarifier 4 remained on track, with the unit expected to return to service in early December.

Clarifiers use gravity to separate solids and suspended particles from wastewater before it flows to the oxidation ponds for final treatment.

The work includes a full overhaul of the drive mechanism powering Clarifier 4’s rotating arms. The council said replacing the system would improve reliability and reduce the risk of further problems as the equipment reached the end of its service life.

The overhaul would also help prepare the clarifier to support the new activated sludge plant when it comes online.

In an earlier update, the council said pond conditions remained significantly better than during the major odour problems earlier this year. However, it warned there could be patches of odour in the coming days and weeks while Clarifier 4 remained out of service.

The council said odour could be worse overnight and early in the morning, when still air allowed gas to accumulate and drift slowly into nearby areas. Oxygen levels in the ponds also dropped overnight because algae stopped producing oxygen in the absence of sunlight.

Residents who notice odour are encouraged to report its location and strength through the council’s Smelt-it reporting tool.

Hourly average hydrogen sulphide readings from community monitors are also available on the council’s monitoring dashboard.