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 to identify 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.