Police have described the scene where three family members died in a water tank in NSW as a "tragic set of devastating occurrences".

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, approximately one hour from Canberra.

It appears the victims were poisoned from carbon monoxide from a "high pressure" petrol engine on a water pump near the tank. Police confirmed there was no drowning involved.

Fumes from the pump are likely to have killed all three, police said.

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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.

The series of events began when a 69-year-old man went into the tank to undergo water maintenance. He collapsed shortly afterwards. The man's 68-year-old brother came into the tank afterwards and collapsed soon after.

The first man's wife, a 69-year-old women, entered the tank after the second man had collapsed.

All three died within a couple hours of each other.

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

"From initial reports there were high levels of poisonous gas in the water tank," Superintendent Andrew Koutsoufis said at the scene.

The bodies had to be left in the tank overnight because of fumes in the tank. Police said they were "not sure" for how long.

"They all did a wonderful job in response to a really nasty incident," Supt Koutsoufis said.

The deaths are not being treated as suspicious.

Police said they were preparing a brief for the coroner.

Names of the family have not been released at this time.

"They need time to grieve," police said of the family.