An oil spill covering about 30km across Darwin Harbour is being investigated by Northern Territory authorities.
The oil is believed to be a "heavy, dark fuel oil", said Dr Bill Freeland, chair of the NT Environmental Protection Authority.
"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 in a statement on Friday evening.
Mr Freeland said the oil has concentrated as a thin sheen towards the centre part of the harbour, but it is constantly moving with the tides.
The Department of Land Resource Management mapped the spill by helicopter as being in several different segments, covering a distance of about 30km.
But Mr Freeland said the spill did not constitute an emergency.
"The sheen is so thin that it is impossible to take any remedial action but it is expected to disperse over time," he said, adding that the INPEX-led Ichthys LNG Project had no involvement in the spill.
The Australian Marine Conservation Society spokeswoman Jacqui Taylor said damage from the spill "could be devastating and cause permanent harm" to Darwin Harbour.
"Darwin Harbour is one of the most important mangrove and marine ecosystems in Australia, supporting amazing wildlife and important recreational and commercial fisheries," she said in a statement.
"This oil spill risks the health of the harbour which has already been hard hit by industrial development, mangrove destruction and pollution.
"AMCS has grave concerns about the potential impact of this spill on our fish stocks, marine life and tourism operators that rely on a healthy harbour."
She said an investigation was necessary to determine the cause of the spill and said those responsible had to be held fully to account.
