The results, released to the Otago Daily Times yesterday, showed a dramatic decline in levels of enterococci in the water and faecal coliform in shellfish along the coastline, including at the popular St Kilda and St Clair swimming beaches.
High levels of the two types of bacteria were good indicators of the presence of other, more difficult to detect, pathogenic viruses in the water and shellfish, Mr Turner said.
The results follow the commissioning of the new $37 million, 1.1km outfall pipe at Tahuna on January 23, which aimed to tackle the persistent problem of high bacteria counts closing the city's beaches.
Between January 28 and March 2, 20 rounds of testing have been carried out at sites along the coastline to check the pipe's performance.
The tests calculated the most probable number (mpn) of bacteria in 100mm water samples, he said.
The results showed entero-cocci levels at St Kilda had dropped from a mean 100mpn before the outfall's commis-sioning, to just 16mpn afterwards.
At St Clair, the level had dropped from 49mpn to 14mpn.
That was well below 280mpn considered safe to swim in, according to Ministry of Health guidelines, Mr Turner said.
Levels at other sites along the coast showed a similar trend, with a Tomahawk location - east of the old Lawyers Head outfall - showing the most dramatic improvement, down from a mean 2901mpn level to just 82mpn.
Levels of faecal coliform found in shellfish showed similar declines at Smaills beach, Lawyers Head, the St Clair Salt Water Pool and Second beach.
The results come days after two St Clair residents told the ODT a plume of pollution and "brown scum" in the water were again visible along the coastline.
However, Mr Turner said the brown scum was a recurring but non-toxic algal bloom, and the plume coming from the outfall pipe on Sunday was largely stormwater that flushed through the pipe following heavy rain.
Dunedin received about 40mm of rain in six hours on Sunday, flushing 3.4 cumecs (cubic metres of water per second) through the outfall pipe for several hours, compared with the pipe's normal 0.5 cumec flow, he said.
Most of the water coming from the new outfall pipe was stormwater that had found its way into the city's wastewater system, despite the two being separate systems, he said.
That could happen for a variety of reasons, including holes in manhole covers to cross-connected pipes, but the problem was being investigated, he said.
Sunday's rainfall had also caused high bacteria counts at two sights - creeks at Tomahawk and near the St Clair saltwater pool - which was also blamed on a variety of sources, from cross-connected pipes to animal discharges finding their way into the creeks, he said.
Those levels were considered unusual and were not included in the data used to calculate mean levels at the two locations, so as not to skew the results, he said.
Overall, the results showed the city's new outfall pipe was working as expected, Mr Turner said.
Coastal currents would continue to push the pipe's plume west towards St Kilda and St Clair beaches about 25% of the time, but the outfall pipe's depth - about 25m at high tide at its end - meant the treated discharge would be dispersed, he said.
Computer modelling showed the plume would travel down the coast and out towards White Island most of the time, rather than towards the beaches, he said.
On the few occasions it washed towards the beaches, the plume's diluted state was not expected to cause a problem.
A plume could continue to be visible during large flows like Sunday's, or for short periods when the outfall pipe was "flushed" daily, but chlorination of the treated discharge - expected to begin in April - was expected to make the plume even less visible and contaminated, he said.
However, "surface tension differences" between the saltwater sea and the freshwater discharge meant some sort of plume was likely to be visible indefinitely, depending on conditions, he said.
