Caterpillar Northern Ireland is likely to face enforcement action after thousands of litres of red diesel poured into the sea from its plant in Larne.
The company and the Northern Ireland Environment Agency (NIEA) have come under fire for a delay in notifying people about the spill, which threatens Larne Lough and The Gobbins.
The storm drain leak was discovered by Caterpillar on Saturday morning.
While environmental authorities said they believed the impact on the sea and marine life would be limited and the clean-up operation has been restricted to the immediate area of the spill and nearby beaches, wildlife groups warned that the slick could have a massive effect.
The diesel has drifted close to a number of wildlife hotspots, including The Gobbins, which is a key breeding site for puffins, and Larne Lough, where Northern Ireland's only pair of roseate terns nest.
Green Party leader Steven Agnew said he was concerned at the apparent lack of communication after an estimated 40,000 litres of red diesel poured into the sea off the Antrim coast. "The diesel continues to drift towards Muck Island and the Gobbins, which are protected areas with large numbers of seabirds currently sustaining hungry chicks with potentially contaminated food," he added.
"I am seeking answers from the minister as to what measures were taken to prevent people from entering the water, whether the NIEA is aware of the impact of the diesel spill on health, what clean-up measures are being taken and whether the costs will be covered by Caterpillar Northern Ireland.
Both the NIEA and Caterpillar are currently carrying out investigations into the cause of the diesel spill.
Staff from the Food Standards Agency have taken samples that will be compared to tests taken before the accident.
It is understood that Caterpillar is likely to face enforcement action and cost recovery down the line because the red diesel should have been contained on site by fall-back mechanisms such as oil bunds (round tanks). These apparently failed.
A major clean-up operation was launched after the spill, along with smaller clean-ups on local beaches.
It is understood that the NIEA is confident that the diesel will evaporate and be quickly dispersed by ocean waves and currents, limiting the long-term impact on wildlife.
However, the RSPB warned that the red diesel could have a massive impact on wildlife. The organisation has brought forward a planned monitoring trip of animals living around Larne Lough.
Caterpillar NI said it has employed the services of an accredited environmental contractor to assist with the clean-up.
"This contractor has worked diligently along the coast since the incident to help reduce any impact and continues to do so," a spokesman for the company added. "We also continue to work closely with the NIEA. We meet or exceed the statutory requirements for bunding.
"The safety of our employees and members of our community and minimising the environmental impact continues to be our main priority.
"Caterpillar Northern Ireland deeply regrets that the incident occurred and is carrying out an investigation to fully understand how this happened and to make any changes necessary to prevent further incidents."