The New South Wales Environmental Protection Agency has concluded that AGL did not maintain the relevant Camden CSG project gas well to the condition required.

"On the evening of 31 August 2014 methane gas was released intermittently from a pressure safety valve on gas well number five at Spring Farm," EPA chief environmental regulator Mark Gifford said yesterday.

"The gas well was being brought back online after an operational shut down when the incident occurred. High pressure gas levels were occurring at the well head while it was being brought online, however due to the build-up of coal fines in a gas pressure sensor at the well head, lower pressure readings were being transmitted to the control room at the AGL Rosalind Park Gas Plant.

"These appeared to be within the normal range to the operators who allowed the gas well to be operated remotely. This resulted in a pressure safety valve activating on several occasions during the evening.

"A high pressure switch that was supposed to shut down the well system when higher pressures occur did not activate on the evening."

Gifford said the EPA received a number of complaints from the Spring Farm community and that the "hissing sound" of the incident would have created concerns for nearby residents.

"To prevent a similar incident happening again AGL has informed the EPA that it is requiring a visual inspection of the pressure gauge prior to the restarting of closed wells and it has revised well start up procedures and enhanced operating and maintenance measures," he said.

An AGL spokeswoman told the Sydney Morning Herald it was important to note that the incident did not cause harm to human health or the environment.

Lock the Gate Western Sydney coordinator Daniel Robins said the $15,000 fine was just a slap on the wrist.