A Top End beekeeper says hundreds of thousands of his bees in 120 hives have died after being poisoned with the loss costing him up to $70,000.

The hives were 15 kilometres to the west of Katherine, and were believed to have been sprayed with an insecticide on Sunday or Monday.

Nathan Woods has notified NT Police about the loss of his bees and offered a $5,000 reward for anyone who has information that can lead to a conviction of those responsible.

He said there was a scene of "total devastation" when he checked the hives on Wednesday morning.

"I arrived and there was just carnage, dead bees everywhere," Mr Woods said.

"I opened the hives up and it was obvious they had been poisoned because all the bees were dead on the inside as well as outside.

"The only way that could happen is if someone sprayed in the hives."

Mr Woods says the poisoning of his bees will cost him about $70,000. ( ABC Rural: Daniel Fitzgerald )

Mr Woods said not only had he lost thousands of bees, but he would also have to destroy the honey and frames in the sprayed hives.

"We will bring all [the hives] home, tip all the frames and burn them, along with the honey in them because we can't use that," he said.

"The boxes we can salvage. We can wash them down, sterilise and repaint them, but the materials inside we can't salvage that at all.

"It will take quite a bit to replace [the bees], as we are always battling bushfires and finding enough food to build up bees.

"To have someone come and do this to 120 hives, it is going to take quite a while to get those hives back and at quite a lot of cost."

Mr Woods says he will have to burn all the frames and honey in the hives that were poisoned. ( ABC Rural: Daniel Fitzgerald )

Mr Woods was at a loss to fathom why someone would kill thousands of bees.

"The only reason I can put it down to is vandals. Someone thought it would be funny I suppose," he said.

"I have had hives run over in the past, I have had them burnt, quite a few get stolen, but I have never had them poisoned before."

Mr Woods, who runs about 800 hives, is one of only a handful of commercial beekeepers in the Top End.

He said the loss of a significant portion of his hives would likely affect the nearby melon industry which relied on bees for the pollination of fruit.

"There are not a lot of beekeepers around and there are quite a few watermelons grown in the area, so it will certainly have an effect on the numbers of bees we can put on farms," Mr Woods said.

NT Police said they have begun making inquiries about the poisoning.