Three people have died in a water tank on a New South Wales farm in Australia.

The two men and a woman, all aged in their 60s, were killed early Thursday evening on the property on Veterans Rd, Gunning, on the Southern Tablelands.

Emergency services were called to the property by a neighbour about 6.20pm after reports of the trio collapsing in the empty groundwater tank.

One man is thought to have collapsed inside the wall and when the second man and woman went inside to help him they too both collapsed.

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When officers from the Hume Local Area Command arrived all three were already dead.

The deaths are not being treated as suspicious.

It's possible the victims were poisoned from a petrol engine on a pump near the tank.

Police said they were preparing a brief for the coroner.