Darwin's helicopter rescue service has launched a mission to reach a worker who became sick on a luxury cruise ship off the Arnhem Land coast.

The CareFlight helicopter, staffed by a doctor and nurse, is travelling about 500 kilometres from Darwin to reach the ship.

Careflight director Ian Badham says the man in his 20s is believed to have a heart problem.

"The ship has a helipad on board the bow of the 328-metre long cruise liner, and hopefully we'll be able to land on that," he said.

"Otherwise the helicopter crew will have to use their winch to evacuate this ill crew member, then fly him back to Maningrida from where he'll be taken to Royal Darwin Hospital."