A prison guard who pretended to be a police officer in exchange for a $10 McDonald's discount has been sacked. 

Lawrence O'Driscoll-Faitaua pleaded guilty in Sydney's Waverley Local Court on Tuesday to impersonating a cop to obtain a $10.53 fast food discount.

The 43-year-old, who worked as a prison guard at the Sydney Police Centre in Surry Hills, admitted to showing his ID card reading "NSW Police - External Contractor" to the drive-through staff at McDonald's Kingsford.

The discount entitled him to 50 per cent off the price of his fast food meal.

A prison guard who impersonated a police officer to get a fast food discount is likely to lose his job for the sake of $10. Lawrence O’Driscoll-Faitaua appeared in Waverley Court today pleading guilty to asking for a police discount at the Kingsford @McDonalds last month. #7News pic.twitter.com/r6ywilHD81

— 7 News Sydney (@7NewsSydney) January 22, 2019

Staff at the McDonald's store photographed the ING bank card he used to pay, and reported the male to the Eastern Beaches Local Area Command. 

O'Driscoll-Faitaua was arrested when he returned to work on New Year's Eve, admitting that it was "common practice" among prison guards to impersonate police officers to receive a discount. 

The Maroubra man allegedly saved $10.53 on a Big Mac and nuggets.

A Corrective Services spokesperson confirmed Lawrence O’Driscoll-Faitaua was sacked following his arrest.

Despite being convicted on a charge of impersonating a police officer, the ex-police guard was granted a section 10 non-conviction in court.