A middle-aged driver caught watching pornography on his dashboard-mounted mobile phone has been hit with more than $1000 in fines.

The 50-year-old man from West Gippsland was caught in the act by an off-duty police officer on the Monash Freeway about 5.30am on November 23.

​Because of the darkness around the vehicle, the officer was able to see the phone's screen while driving next to the car and took down the details of the driver.

The man was interviewed by police on Saturday. Police said he made full admissions and was handed two fines.

Sergeant James Robbins said it was an unusual and unacceptable case.