Two men and woman, all believed to be in their 60s, died while in an in-ground cement water tank on a rural property near Gunning last night.
Police understand one of the men collapsed while working in the tank at the property on Veterans Road, leading to the second man and the woman attempting to assist him before collapsing themselves.
A neighbour contacted emergency services at around 6.20pm.
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Officers from The Hume Local Area Command established a crime scene which will be examined by forensic officers, but police are not treating the three deaths as suspicious.
The Yass Tribune reports that investigators conducted gas detector tests at the scene last night, finding high levels of CO2.
A brief will be prepared for the information of the Coroner.