A large oil spill in Darwin Harbour has authorities scrambling to respond.

Northern Territory Environment Protection Authority (NTEPA) chair Dr Bill Freeland says a “heavy, dark fuel oil” was spilled, “either from some fuel oil storage or perhaps from a largish ship but we can't be certain as to what it is at the moment”.

“It is behaving oddly and it may have been contaminated with a dispersant, suggesting whoever did it may have put some dispersant in it.

“Consequently, it's formed a very thin sheen that is moving very rapidly up and around the harbour, backwards and forwards.

“It is essentially contaminating a fair chunk of the harbour.”

Dr Freeland indicated there was not much the EPA could do.

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

“Parks and Wildlife staff are on standby in case there are any oil birds, but we haven't found any yet.”

Members of the Australian Marine Conservation Society have collected water samples for testing.