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

It is understood a man had collapsed while working in an empty in-ground cement water tank.

Police have been told a second man and a woman went to his assistance, but also collapsed.

A neighbour contacted emergency services.

However, the bodies of the two men and the woman, all believed to be aged in their 60s, were found dead inside the water tank.

It is believed investigators are looking at whether emissions from a petrol engine, possibly a pump in or near the well, contributed to the deaths. Just after 8pm, a gas detector had found high levels of CO2.

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.

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. 

The Oolong property is just off the Hume Highway, about one kilometre from Gunning.