Environment Waikato and Hamilton City Council staff are investigating the source of a diesel spill in the Waikato River, which appears to have originated near the Fairfield Bridge in Hamilton.
Downstream industrial users of river water, such as Affco and Fonterra, have been alerted.
It is unclear at this stage exactly how much diesel has gone into the river or where it has come from, but signs of the diesel have been spotted near Fairfield Bridge, at Pukete and at Te Rapa.
"It appears to have been a reasonably significant spill," said wastewater resource officer Mark Row.
He said Hamilton City Council staff were checking the stormwater system for clues as to where the diesel may have come from.
At this stage, there was not a lot else that could be done to respond to the spill other than alert the downstream industrial users, Mr Row said.