Council director of resource management Selva Selvarajah said the negative test results - obtained this week - confirmed only that plumbers had fixed the problem which had caused the Orchard Grove creek to become contaminated with human waste.
Neighbours complained to the Dunedin City Council last month after discovering a plumbing mistake had resulted in sewerage pipes from two homes being connected to stormwater drains.
The errors, which occurred 18 months and six months ago, went undetected until last month, during which time raw sewage fouled the creek, a popular playing area for neighbourhood children.
Regional council staff took water samples only last week, days after plumbers had returned to fix the faults.
Dr Selvarajah said staff were also still compiling a report into the circumstances surrounding the contamination, which was expected to be completed by the end of next week.
Any decision to proceed with a prosecution would be made then.
Last week, Dr Selvarajah criticised the failure to detect the plumbing fault and warned the city council and the plumbing company were among parties which could face prosecution.
