Oil has been located in water off the coast of Townsville along the Great Barrier Reef.

An helicopter spotted the oil this afternoon after an earlier sweep by a Customs aircraft failed to locate any following reports from a commercial vessel.

Queensland Water Police also dispatched a vessel to the site and ships in the vicinity of the spill were asked to keep a lookout.

Early tomorrow morning, a maritime safety aircraft will head out to the area to measure the extent of the slick path.