Dunedin City Council staff have confirmed levels of faecal contamination have come close to closing St Clair beach in the months since the $37 million Tahuna outfall pipe was completed.
However, rather than coming from the city's treated pollution being pumped out to sea, the plume was from the city's stormwater system, council water and waste services manager John Mackie said.
The problem was at its worst during periods of heavy rain, when contaminants from septic tanks and run-off from properties could be flushed through the stormwater system, he said.
The heavy rain could cause sewer systems to overflow into stormwater systems, as rainfall could push the flows through wastewater pipes at rates as high as 13 times their normal volumes, he said.
"If the pipes aren't designed to take that load, you will get spill."
The wash was making its way through the city's stormwater system to outfall pipes emptying into the sea or Otago Harbour, and the problems worst at St Clair, Mr Mackie said.
Late last month, staff testing the beach had found enterococci levels at 270 per 100 millilitres of water, just 10 below the safe level of 280, although tests the next day found the contamination had dispersed.
Mr Mackie said the plumes of contaminated stormwater at St Clair were more localised than plumes that came from Lawyers Head before the commissioning of the new outfall pipe.
However, it would "certainly not" be advisable to swim at St Clair if the levels reached 280.
The problem was neither new or unique to Dunedin, but the completion of the Tahuna outfall pipe in January had revealed the extent of other contaminants finding their way to the city's beaches, he said.
Infrastructure services committee chairman Cr Andrew Noone said much of the contamination at St Clair was coming from the higher ground of Caversham and Corstorphine.
The council was continuing to monitor outfalls for contamination, and "pretty much all" of them were showing clear results, Cr Noone said.
"We would like to think we have got all that behind us, but obviously the discharge at this one particular point is pushing the boundaries a little bit."
There had been no beach closures since the pipe's commissioning on January 23, and the council was also in the design stage for stage two of the project, the $74.3 million secondary treatment programme at the Tahuna plant.
Cr Noone said city council and Otago Regional Council staff were discussing long-term solutions, including a new filtration system and the control of contamination at its source.
Mr Mackie said repairs to the worst parts of the city's sewer network, underneath Fryatt St, were planned for early next year, and other repairs would follow as part of the city's ongoing renewals programme.
