Carbon monoxide poisoning is suspected to have killed three family members found dead at the bottom of a concrete water tank on a rural property in the New South Wales Tablelands.

Andrew, Anne and Richard Basnett all died after they each entered the tank late yesterday afternoon.

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.”

Police said a 69-year-old man was using a high-pressure water pump to clean inside the tank Thursday evening on his property in Gunning, near Yass, when he collapsed.

The man’s brother, 68, and the man’s wife, 63, jumped into the tank to try to help, but also collapsed.

A neighbour who heard the woman’s cries for help found the three bodies and called emergency services.

NSW Police Superintendent Andrew Koutsoufis said the deaths appeared to be a tragic accident.

“There were high levels of poisonous gas in the water tank by the time police got there,” he said.

“A petrol [water] pump being used the clean the tank … may have been the cause of the build up of fumes at the bottom of the tank.”

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

He said post-mortems would be needed to confirm the cause of death.

“Carbon monoxide poisoning is a line of enquiry we are following,” he said.

Police said the family of the deceased had been told.

“They just need some time to grieve,” Superintendent Koutsoufis said.

“It’s devastating for a small community like Gunning. A lot of people are hurting right now.”

Forensic officers earlier established a crime scene, despite the deaths not being treated as suspicious.

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

The Gunning property is north-west of Canberra. Photo: Channel Nine

The police from Goulburn had joined emergency services counterparts at the scene, including ambulance and fire and rescue personnel from Yass, and the State Emergency Service from Queanbeyan.

Gunning cafe worker Kirsty, 20, told Channel Nine the deaths had rocked the local community.

“It’s major news in the town especially in such a small place like this,” she said. “Everyone knows each other and it really hits home.”

Police said post-mortem examinations, to be carried out over the coming days, would help piece together what happened.

– With ABC