A judge has called a West Coast mining company "careless" for discharging diesel into a Waikato stream.
Taharoa Ironsands leaked 4000 litres into the Wainui Stream though a mining site stormwater system last year.
One local resident tracked the fuel 400m out to the Tasman Sea.
Te Kuiti District Court judge Melanie Harland says there was a lack of communication and a failure to follow procedures. She called the company's actions "careless".
"This was completely avoidable. Companies that use and store hazardous substances have to ensure they have good infrastructure and procedures in place.
"Accidents can happen. When they do, substances like fuels and oils need to be contained so they do not escape into the environment and cause harm."
Taharoa Ironsands was fined $60,750.