An industrial area of Te Rapa was the source of unauthorised discharges of diesel into the Waikato River.
Waikato Regional Council compliance and education manager Rob Dragten said immediate steps were put in place to prevent any further spills into the city's stormwater system.
He said it was pleasing toidentify the likely source of recent unauthorised spills and the organisation identified had been co-operating.
"But as this investigation is ongoing, with legal action possible, it would be inappropriate to identify the source," Mr Dragten said.
"There is a complex maze of stormwater drains which lead to the outlet near St Andrews Golf Course and so finding the source of the spill has been an incredibly difficult task.
"We have worked closely with Hamilton City Council to identify the source. Staff from both councils have been lifting manhole covers, assessing drains in the catchment, as well as using CCTV inside the stormwater system.
"This has enabled us to track back up the network of pipes to find the point of discharge."
He said numerous samples were taken from several locations and would require further testing.
"While there is no longer any diesel discharging from the outlet, there is a possibility a small amount might be sitting in the stormwater system, which is likely to be flushed out when it next rains," Mr Dragten said.
The latest report of diesel discharging into the river from the Arcus St outlet was the fifth in seven months.
While it was hoped there would be no further spills into the river, this was a good reminder of the need for businesses to prevent toxic discharges from entering the stormwater system and ultimately the Waikato River, Mr Dragten said.
