Three members of the one family have died following an incident on a rural property in the state's Southern Tablelands overnight. Another man and a woman climbed down into to the tank to try and get the man out but they also collapsed.
According to the Canberra Times, investigators are looking at "whether emissions from a petrol engine, possibly a pump in or near the well, contributed to the deaths".
A gas detector has found high levels of Carbon dioxide in the tank.
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Police are expecting post mortem examinations, to be held in coming days, to help determine what happened.
ABC News reported the three people were all from the same family.
It's believed that after 69-year-old man collapsed while working in an empty in ground cement water tank, his bother, 68, and wife, 63, went to his aid in the tank and also collapsed.
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The neighbour was able to contact emergency services.
Emergency service personnel worked long into the night attempting to pull the victims' bodies from the well.
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.
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Post-mortems will confirm the cause of death and police will prepare a report for the coroner.