As Associate Professor Stuart Khan from the University of NSW notes, the four main chemicals – benzene, toluene, ethylbenzene and three forms of xylene – occur naturally in coal seams, although amounts vary across deposits.

Natural or not, authorities don't want BTEX chemicals turning up in drinking water.

Benzene is a known carcinogen and Australian drinking water guidelines - which Dr Khan helped to revise - restrict it to 1 part per billion (ppb) for water intended for human consumption. The other three are contaminants but are not considered cancer-causing, and so authorities permit higher concentrations.

AGL denies using BTEX in its fracking - the process of pumping sand and fluids into coal seams to free up the gas. The government will want to confirm that.

"The reported concentration of 555 ppb [of BTEX for one AGL sample] is very high and probably the highest concentration I have heard of in environmental water," Dr Khan said.

"But flowback water is not typical environmental water, and so it is not necessarily unexpected."

Dr Khan said this concentration is unlikely to be maintained in large volumes of produced water if CSG wells become operational.

He also noted that the permitted concentration levels are based on very conservative measures, such as the amount of water somebody would drink over a lifetime. People are likely to face much higher exposure levels – albeit briefly –  when filling up at the service station.