Mr Thrush said complaints about the odour had now extended from Mt Herbert Rd, which runs past the oxidation ponds, to the likes of Tavistock Rd, Woburn Rd and Porangahau Rd, near the old hospital.
The council had also considered other possible sources such as nearby farming operations, problems with people's individual septic tanks not draining properly in the wet conditions, and flooded paddocks causing grass to rot, he said.
Residents spoken to, however, said the odour wasn't vegetable or agricultural in nature - variously describing it as a sewage smell, or sulphuric, similar to that in Rotorua.
Jo Brabyn lives on Mt Herbert Rd, and said it had been happening almost every night and seemed to have worsened with winter approaching.
She said she was sure it was the oxidation ponds.
"It's a very strong, foul smell, it's not sweet like silage, it's a sewage smell."
Linda Greer also lives in Mt Herbert Rd and said this summer was the worst time, when in the heat of the day her family had to close the windows to keep the "horrendous" smell out.
She said the wind direction dictated who caught the whiff, and wondered if the dying out of the predominantly westerly summer winds might account for people further afield now noticing it.
"The smell is always here, it just depends on what the wind's like as to who is affected by it."
Born and bred in CHB, Emma Mason-Smith had lived in Mt Herbert Rd near the ponds for the last eight years and operates a hairdressing business from her home.
She said she was insulted that the council was saying it was looking for the source of the issue, especially as council representatives had not talked to residents despite complaints being received for many months.
"We want the council to be transparent. We know the odour is coming from the pond.
"Those ponds with the new [floating wetlands] treatment have not worked - the council has used ratepayers' money and it has not worked - they have to put their hands up."
CHB council chief executive Monique Davidson said the council recognised that its communications with residents about the matter had not been good enough.
"We will be focused on improving that over the course of this week - we understand that it's frustrating for residents but we are committed to finding a solution.
"We appreciate residents' patience and understanding over this longstanding issue."
She said the intent of the survey was to get as much information as possible about factors affecting the odour such as the location, the time of day and weather conditions to get as bigger picture as possible of the problem.
In addition to laying covers over open channels from the anaerobic pond, the outlet structure would also be covered, the pipe outlet would be sleeved and planned landscaping around the ponds should help to act as a natural barrier.
"We are also looking to install a flare at the site that will burn off gases from the anaerobic pond.
"The biggest issue is trying to isolate where the odour is coming from and we are taking a multi-pronged approach to that."
