Caterpillar NI said the diesel had got into a storm drain.
The Northern Ireland Environment Agency has launched an investigation.
Sinn Féin MLA Oliver McMullan is calling for a full investigation to be carried out.
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Mr McMullun said: "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.
"Our coastline in East Antrim is one of our prime assets in terms of tourism and it must be protected.
"I will be contacting the Minister for Agriculture, Environment and Rural Affairs to ask exactly how this happened and what steps can be taken to ensure it is not repeated."
