'It's devastated a family ': Husband, wife, brother killed in freak water tank tragedy

Two brothers and one of their wives have been found dead in an empty water tank on a NSW Southern Tablelands property after being overcome by fumes in what police say is a "tragic accident".

Authorities suspect carbon monoxide poisoning is behind the deaths of married couple Andrew and Anne Basnett and the man's brother, Robert.

It's understood Andrew, 69, got into the empty in-ground cement water tank on the farm in Gunning to carry out maintenance work but he quickly lost consciousness and died.

His brother, Robert, 68, then jumped into the tank to check on him, but he too succumbed to the fumes and collapsed.

Married couple Andrew and Anne Basnett (left) and the man's brother, Robert (right) were tragically killed by what is thought to be a freak carbon monoxide accident.

The trio is believed to have succumbed to carbon monoxide poisoning inside the Gunning farm's water tank. Source: 7 News

"There was a petrol pump being used to clean the tank and that may have been the cause of the build of fumes at the bottom of the water tank," NSW Police Superintendent Andrew Koutsoufis told reporters on Friday.

Andrew's wife Anne then alerted a neighbour before also climbing into the tank to check on the collapsed pair before she too was tragically killed by the fumes.

"It's an absolute tragic set of circumstances, a tragic accident," Supt Koutsoufis said.

"It's devastated a family ... and being a small country town, it's also a devastating outcome for a small town like Gunning. So a lot of people are hurting at the moment."

The close-knit town of Gunning has a population of about 450.

The freak accident has left a family and small country town absolutely devastated.

The trio were discovered by the neighbour who came to check on them and contacted emergency services just after 6pm on Thursday.

The bodies remained in the tank overnight as rescue specialists waited for the gases to dissipate, Supt Koutsoufis added.

Police said autopsies would be carried out in coming days.