The husband Andrew, 69, was conducting routine maintenance on the cement tank when he lost consciousness.

His 60-year-old brother, Robert, then went into the tank to check on him and also passed out.

Shortly after, Andrew's wife, Anne, entered the tank and subsequently collapsed.

Police believe he was using a petrol pump, which may have caused a build-up of fumes at the bottom of the tank.

Authorities found all three of the family members dead inside the tank after a neighbour raised the alarm.

There was a petrol pump being used to clean the tank and that may have been the cause of the build up fumes at the bottom of the water tank. Carbon monoxide poisoning is the most probable cause of death.

Occupational physician Associate Professor Anthony Brown of the University of Sydney said confined spaces can often pose a considerable safety risk due to poor ventilation.

"In confined spaces, you have a limited atmosphere and it’s easy to contaminate it," he said.

"The sort of thing that often happens is you have a contamination and levels get high enough, causing someone to collapse.

"Someone else sees it, they go into to help, which is quite laudable -- but then they’re affected too."

Prof Brown said that the colourless, odourless nature of carbon monoxide often means its presence goes overlooked, until too late.

Prof Brown added that confined spaces "need to be taken seriously" as they are "potential dangerous all the time".

"If you're going into a place that’s closed up without ventilation you have to think seriously, 'How can I do this safely?'" he said.

Procedures for entering confined spaces

To raise awareness among employers and workers about the importance of adequate health and safety training, and safe work procedures for entering confined spaces.

This video documents the Cody McNolty story.

On October 4, 2001 Cody McNolty was working as a welder's assisstant in northern British Columbia. He was helping his father refit a log barge. Precious Time documents Cody's story, a story of confined space rescue that goes tragically wrong.