Caterpillar Northern Ireland said the diesel had got into a storm drain which runs to the sea off Larne.
It is believed around 40,000 litres of fuel was involved. It was part of a bigger spill, some of which was contained on site.
The incident is being investigated by the Northern Ireland Environment Agency (NIEA).
People involved in a regatta off the County Antrim coast on Saturday reported a long diesel slick and heavy fumes.
It is believed the slick spread several miles north of Larne harbour.
Caterpillar said it had reported the incident when it was discovered.
It said it had also employed an environmental contractor to help with the clean up.
A spokesman for the firm told the BBC: "A control unit that manages the quantity of diesel entering a test tank failed.
"We are carrying out our own investigation to fully understand how this happened and to implement a permanent remedy."
The NIEA has taken samples with a view to enforcement.
East Antrim MLA Oliver McMullan, from Sinn FÃ©in, called for a full investigation.
"This major diesel spillage off the coast of Larne is a cause for serious concern in the East Antrim area.
"The company behind the spillage have said it was responsible but a number of questions still have to be answered.
"We need to know how this diesel got into storm drains and ended up in the sea," he added.
Caterpillar Northern Ireland mainly makes diesel generators at its Larne facility.
A leading manufacturing firm has said it was the source of a big pollution spill off the County Antrim coast.