NSW farmers are being urged to be aware of the dangers of using pump or motor engines in water tanks following the deaths of three family members in the Southern Tablelands.
Andrew Basnett, his wife Anne and brother Richard died from suspected carbon monoxide poisoning after they entered the tank at a property outside Gunning, near Yass, on Thursday afternoon.
Andrew, 69, was using a high-pressure water pump to clean the empty in-ground cement tank when he collapsed.
Richard, 68, and Anne, 63, suffered the same fate when they went to help.
Carbon dioxide fumes are believed to have built up in the tank as the water pump was used.
The invisible, odourless gas can be deadly in confined spaces, NSW Farmers president Derek Schoen says.
"The gas is much heavier than the surrounding atmosphere so it sinks straight to the bottom of the tank," he told AAP.
"Once that happens, it slowly moves up towards your head and then you asphyxiate."
The tragic circumstances surrounding the Basnett family deaths were not uncommon, he said.
"The natural instinct when you see someone collapsed is to go in and help but that is the worst thing you can do," Mr Schoen said.
"It only takes about a minute and a half to asphyxiate so there's not much time to get out of there while carrying a heavy body."
The best thing to do is call emergency services, which have the right breathing apparatus and retrieving methods.
Mr Schoen also urges water tank owners to place pumps or other engine equipment further away.
"As far as possible. Not just with tanks but underground wells too," he said.