The three family members were found dead in the concrete water tank at a property near Oolong, between Yass and Gunning, believed to have collapsed from a gas present inside the tank.

The farmer collapsed while working in the empty cement water tank. His wife and brother went to his assistance, but also collapsed.

His wife is believed to have called neighbours first, who contacted emergency services.

The bodies of the three, all aged in their 60s, were found inside the water tank.

The family members, who Fairfax Media have chosen not to name while other family members continue to be informed, were described as "pillars of the community" and "good country people" by devastated neighbours.

Acting Superintendent Andrew Koutsoufis of Hume LAC said the cause of death couldn't be confirmed at this time but investigators were looking into the presence of Carbon monoxide as a line of inquiry before preparing a brief for the coroner. 

"There was a petrol pump being used to clean the tank and that may have been the cause of the build-up of fumes at the bottom of the water tank," he said. 

"It appears that it was lowered into the tank and that's caused the gases to be at the bottom of the tank." 

He said the teams called to the scene had to wait for gas levels to recede before retrieving the bodies.

Superintendent Koutsoufis said the "close-knit town" was "hurting at the moment" and the family that lost three relatives in the sudden and tragic accident were devastated. 

Neighbours said the couple had farmed the property - called Nerragundah - since the early 1980s at least and their contribution to the town of Gunning would be sorely missed.

They leave behind three adult children, living in Sydney , as well as a number of grandchildren.

Deputy Mayor and close family friend, Kim Turner paid tribute to the couple.

"It's a massive loss for the community. They were great community people, and the loss is insurmountable, not only in Gunning, but throughout the region.

"I personally will miss both of them enormously."

"We are devastated. It is safe to say we are all in shock. They were lovely people," said one neighbour who did not wish to be named.

Police and other emergency services were called to the rural property, just off the Hume Highway, on February 16 about 6.20pm.

Emergency services personnel retrieved the bodies from the tank near the front of the property just after 9pm.

The deaths are not being treated as suspicious. A brief will be prepared for the information of the Coroner, as officers from the Hume Local Area Command have established a crime scene to be examined by forensic officers.