"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.