The Northern Territory Environmental Protection Authority and other agencies are continuing to investigate today's significant oil spill in Darwin Harbour.

The chair of the NT EPA, Dr Bill Freeland, said the oil was believed to be a "heavy, dark fuel oil".

"If it's a fuel oil, it is either from some fuel oil storage or perhaps from a large ship but we can't be certain as to what it is at the moment," he said.

It has concentrated as a thin sheen towards the centre part of the harbour, although it is constantly moving with the tides. The Department of Land Resource Management had mapped it by helicopter as being in several different segments, covering a total distance of 30km.

Dr Freeland said that the NT Oil Spill Contingency Plan had not been activated because it was not an emergency.

The sheen is so thin that it is impossible to take any remedial action but it is expected to disperse over time.

Dr Freeland said that INPEX-led Ichthys LNG Project had no involvement in the oil spill and had been collaborating with the NT EPA by providing a vessel for the EPA’s harbour inspection.

Darwin Port provided a pilot boat and spill equipment; the Water Police assisted by modelling potential water movement.

 People are urged to call Marine Wildwatch on 1800 453 941 or go to https://nt.gov.au/environment/environment-data-maps/marine-coastal-biodiversity if they notice sick or injured wildlife while out on the water in Darwin Harbour this weekend.

For further information: Helen Farquhar 0437 458 213