Scientists are stunned after the largest ever pod of killer whales in Australia's northern waters was filmed by a fisherman off the NT coast.

The pod of 15 killer whales, or orcas, was seen by fisherman Luke Burton and a friend on Friday last week at Fog Bay about 45 kilometres from Dundee, which is south-west of Darwin.

"That footage is fantastic, the way they kind of are checking out Luke on his boat and chasing Luke actually," said Department of Land Resource Management senior scientist for marine mammals, Carol Palmer.

Ms Palmer said orcas, which are actually the world's largest dolphin and not a whale at all, are not endangered, but tropical orcas were extremely rare.

Two orcas near a fishing boat near Darwin. ( Supplied )

There has only been four sightings of orcas off the NT coastline in the past two years, but the pod recorded by Luke was far larger than the pods of six animals seen previously.

"Potentially the ones in the Northern Territory, or the ones that we are getting across Northern Australia - because these guys probably travel from Queensland to WA I would say - is that they are sort of a genetically distinct population from the ocean-going ones," Ms Palmer said.

Efforts are underway to tag Australia's tropical orcas to find out whether they migrate to colder waters near Antarctica or stay in warmer waters.

Mr Burton, who filmed the orcas with his phone while on a fishing trip, said he saw the fins in the distance and took his boat close to investigate.

An orca spotted breaching. ( Supplied )

"She was pretty exciting, it got your blood pumping a bit. A bit of adrenalin," Mr Burton said.

He said the orcas were joined by a bottlenose dolphin, and they went under and around his 5.5-metre boat.

"Even when they were next to boat we couldn't really believe it," he said.

"The smaller ones would've been at least two metres long and the larger ones were getting up to six or eight metres I reckon," he said.

Ms Palmer urged anyone who spotted orcas off the NT coast to call the Marine WildWatch line on 1800 453 941.