Husband and wife Andrew and Anne, and Andrew's brother Richard, were found dead inside a concrete water tank at their property just off the Hume Highway, near Oolong, between Yass and Gunning.

Andrew, 69, had collapsed while cleaning the empty cement tank with a motorised water pump.

Anne, 63, and Richard, 68, went to his assistance but also collapsed.

They are all believed to have been poisoned by carbon monoxide gas that built up in the tank as the water pump was used.

Before entering the tank, Anne is believed to have called neighbours, who then contacted emergency services about 6.20pm.

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

Acting Superintendent Andrew Koutsoufis of Hume Local Area Command said the cause of death was being investigated.

"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 who lost three relatives 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.

The event, in which the couple were finalists, has been cancelled as a mark of respect.

The Basnett's were also involved in the Gunning Garden Club and a number of other community organisations.

Deputy mayor and close family friend Kim Turner paid tribute to the couple on Friday.

"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.

"They were great community people. Annie was involved in a theatrical group and Andy played golf with us each week."

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

Yass Valley Council Mayor Rowena Abbey and family friend of the Basnetts said the region was in a state of shock.

"This is an extraordinarily sad day for many of us as their friends," she said. "Andy and Annie were wonderful people, full of laughter and passion for life. They loved their farm, their friends and particularly their children and grand children.

"The friends of the family request that Andy and Annie's children be allowed time to try to come to terms with this enormous loss and be given space to grieve."

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.

Acting Superintendant Andrew Koutsoufis of Hume LAC details what is known about the Oolong/Gunning water tank triple fatality overnight. More here - http://www.yasstribune.com.au/story/4474093/three-people-dead-in-water-tank-tragedy-near-gunning/?cs=1554Posted by Yass Tribune on Thursday, 16 February 2017