Around 20 cars have broken down on the M4. It's believed dodgy fuel from nearby petrol station caused the breakdown. Picture: Twitter/Channel 9 Source: Channel 9

The cars filled up at the Caltex service station on the city-bound side near Prospect before being forced to pull into the breakdown lane between Eastern Creek and the Prospect Highway.

The vehicles have been moved to safety and a spokesman from the Transport Management Centre said that Thursday peak-hour traffic had not been affected.

A Caltex spokesman told news.com.au that it would fully reimburse customers for any out-of-pocket expenses incurred as a result of the dodgy fuel.

“The issue is being investigated by our technical team and it appears that water may have entered the tanks. The affected pumps will remain online until the issue has been rectified,” he said.

Caltex has apologised for the inconvenience to customers and said the fuel issue was only impacting the service station on the eastbound side.

“It appears to be isolated to the E10 product at this site only - there are no other products affected,” the spokesman said.

E10 fuel looks to have been contaminated. Source: News Limited

He confirmed Caltex was aware of about 12 cars which had broken down after filling up with E10 petrol, but there could be as many as 20 vehicles impacted.

“We quickly shut down the pumps for that product so no other customers will be affected,” he said.

“Our technical team is on its way to investigate what has caused this issue.”

Caltex is advising customers with any problems to hold onto their receipt and call its customer service team on 1800 240 398.

NRMA spokesman Peter Khoury said its teams started getting calls from customers requesting roadside assistance from about 5am.

Cars being towed off the M4 this morning. Picture: Twitter / Channel 9 Source: Channel 9

“We received up to a dozen calls and we think twice as many cars were affected,” he said.

Mr Khoury said all that could be done for impacted motorists was to tow the cars to a mechanic for repair.

“If the fuel is contaminated, the petrol tank will likely need to be removed out of the vehicle, cleaned and any other parts of the car that have come into contact with the fuel will probably need to be replaced so it will probably be very expensive,” he said.

Breakdowns from contaminated fuel were actually not uncommon he said and tended to happen after heavy rain.

“If there are cracks in the tank, water can seep in and unfortunately the first time you will notice is when a car breaks down,” he said.

- with AAP