The gas leak was first noticed by staff at 11.18am.

South Australian Metropolitan Fire Service (SAMFS) regional manager west coast Darryl Horsell said the staff reacted quickly after smelling gas and operated the emergency shut off and called 000.

He said firefighters reached the site in breathing apparatus and examined the gas cylinder.

"Firefighters using gas detection equipment confirmed that LPG was leaking from valve work associated with the underground cylinder," Mr Horsell said.

"Police set up an exclusion zone around the service station and began evacuating the immediate area downwind.

"Firefighters isolated all potential sources of ignition and as the gas was dispersing downwind towards the two fast food outlets and police evacuated those as well.

"The leak was eventually traced to a specific valve and the firefighters were able to isolate it to the extent that stopped the escape of gas."

MFS incident controller senior firefighter Andrew Higgins said LPG is safe when stored and used appropriately but as it is heavier than air it tends to flow along the ground and settle in low spots.

He said because of this water sprays were used to disperse the gas to prevent it settling and forming pockets.

"Because the gas in its natural state is odourless and colourless a sulphur based chemical was added during manufacture to give it a characteristic smell so that even the smallest amount can be detected,"Mr Higgins said.

Onesteel and Santos supplied the MFS with additional gas detection equipment which allowed the fire fighters to quickly ascertain the flammable vapour range over the entire area.

Firefighters took about four hours to return the situation to normal and technicians from Adelaide travelled to Whyalla to repair the valve.