The Northern Territory Environment Protection Authority's (EPA) response to an oil spill in Darwin Harbour has been "dismal," according to the peak body for recreational fishers.

The spill of a fuel oil was reported to Darwin Port at 4:00am on Friday.

The Amateur Fishermen's Association of the Northern Territory (AFANT) chief executive, Tristan Sloan, said no government agency had contacted the association about the spill.

Authorities said the slick was difficult to contain. ( Supplied: Department of Land Resource Management )

"The communication process around it, from the NT EPA and authorities involved, really has been dismal to say the least at this stage, and we'd like to see that improved in the future," he said.

"When these type of things happen, there needs to be immediate action happen to contain it and control it and find the culprits."

Mr Sloan described the lack of action from authorities as "very poor communication".

"Though, I'm certainly not in a position to judge at this time as I am not aware of all the facts, it seems they were slow to respond to it as well," he said.

NT EPA chair Dr Bill Freeland declined to respond to Mr Sloan's comments.

On Friday, Dr Freeland said the way the oil had formed on the surface of the water meant it would be difficult to mitigate.

"In terms of containment, there are no really effective ways of containing fine sheens of oil spread over large areas," he said.

Authorities have not provided any update on spill today. They are continuing to investigate where the oil came from.

Mr Sloane said fishers believed the oil was from a ship.

Water from the Darwin Harbour spill (L), alongside water from Dinah Beach boat ramp. ( Supplied )

"If it has been identified as coming from a ship, certainly Darwin Harbour and the Harbour Master under ports authorities really has quite a lot of intelligence on the window for large scale shipping traffic leaving and entering the port.

"It would be fairly easy to identify within 24 to 48 hours what ships were in and out of the Harbour."

The Australian Marine Conservation Society northern marine campaigner Jacqui Taylor went out on the harbour in a boat today to collect water samples for testing.

Ms Taylor said she was concerned about the implications of the spill for marine life.

"We are not sure of the extent of it but you know there is a good chance it could affect commercial and recreational fisheries, and we are also very concerned about any animal life and marine life that live within the harbour," she said.

She said the spill demonstrated the need for a plan to protect Darwin Harbour.

"Now is the time to act. We have already seen a range of threats to the harbour we are getting a much larger increase in shipping especially as INPEX is completed, there is going to be about 400 boats moving in and out of here a year."