Three people were found dead pn Friday in a water tank at a rural property in Australia's New South Wales state, police said.
His wife is believed to have called neighbours before entering the tank, who then contacted emergency services.
NSW Police Superintendent Andrew Koutsoufis said the deaths appeared to be a tragic accident.
Police were called to the Gunning property, around 60kms north of Canberra, just after 6.20pm, after a man collapsed while working in an empty in ground cement water tank.
All three of the victims, believed to be aged in their 60s, died inside the tank.
The man's brother, 68, and the man's wife, 63, jumped into the tank to try to help but also collapsed.
"Carbon monoxide poisoning is a line of enquiry we are following", he said.
"There were high levels of poisonous gas in the water tank by the time police got there", he said.
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"They need time to grieve", police said of the family.
"They all did a wonderful job in response to a really nasty incident", Supt Koutsoufis said.
Deputy mayor and close family friend Kim Turner paid tribute to the couple.
"I personally will miss both of them enormously".
Emergency services personnel retrieved the bodies from the tank near the front of the property just after 9pm.
When officers from the Hume Local Area Command arrived all three were already dead.