Waikato Regional Council hopes a hefty penalty on a mining company over a diesel spill will deter other firms from making similar mistakes.
Taharoa Ironsands Limited has been fined $60,750 for leaking 4000 litres of diesel into the Wainui Stream near Kāwhia last year.
Action was taken after the regional council received multiple complaints of contamination near the company's mine site.
One local said they had tracked the rainbow coloured oil slick about 400m into the Tasman Sea.
A subsequent investigation by the council found Taharoa Ironsands had bought large industrial generators and fuel storage bladders to increase their electricity supply.
"There was a series of mishaps that meant that one of the fuel cells was, in effect, open," said the council's investigations and incident response manager, Patrick Lynch.
"It drained diesel down into the immediate area... down into the site's storm water system, went to a very rudimentary oil trap, overwhelmed that, and then flowed directly into the Wainui Stream."
The fine was imposed in the Te Kuiti District Court by Judge Melanie Harland.
Mr Lynch said the situation could have been avoided if proper safeguards had been in place.
"We appreciate that sometimes there can be genuine accidents, and responsible companies need to ensure that there are safeguards in place so that when those accidents do happen, they are contained," he said.
"The lack of containment... where this fuel was kept in these bladders - that was absolutely one of the contributing factors to this."
Mr Lynch said he hoped the hefty fine would deter other companies from making the same mistake.
"If you are storing or using any potentially harmful contaminants, such as fuels and oils and those sorts of things, you have to apply that `well what if this happens? What if that happens'?" he said.
"This is a conviction for this company, and it's a hefty fine.
"We urge any company out there that is storing and using these types of substances to look at their systems and the procedures that they've got in place and safeguards they've got in place and make sure that they are absolutely fit for purpose."
A spokesperson for Taharoa Ironsands could not be reached.