An air and water search is underway north-east of Darwin for three occupants of a tinny who activated a distress beacon this morning.

A water police launch is heading out to support the CareFlight Top End rescue helicopter in the search.

The Australian Maritime Safety Authority in Canberra called the search after aircraft in the area reported hearing the beacon signal coming from an old 121.5 MHz unit.

The pilot, air crewman and flight nurse on the CareFlight rescue helicopter are sweeping an area 45 km north-east of Darwin.

AMSA said the EPIRB beacon is registered to a 5.3m tinny which set out yesterday for an overnight fishing trip to Buffalo Creek.