New South Wales Police Minister Troy Grant says he has learnt his lesson after being fined and losing four demerit points for holding a mobile phone while behind the wheel.

The Nationals MP for Dubbo reported himself to the police after taking a photo of a sheep in the boot of a nearby car while stopped at roadworks at the weekend.

He uploaded the image to Twitter.

"This serves as a massive reminder, nobody is above the law even if you are the Police Minister, even if you have to ring up and dob yourself in to get a ticket," he said.

"If that is what it takes to continue to ram home the message, then that is what I am prepared to do."

Troy Grant is in hot water after posting a photo on social media while driving. ( AAP: Lukas Coch )

Mr Grant said he did not know he was doing the wrong thing by holding a mobile phone while in the driver's seat.

"I didn't know that what I was doing was against the law, I was stationary [and] stopped, using my phone just to simply take a photo, as I was sitting there for a long time at a road work site," he said.

He was fined $325.

"It's a significant penalty, as it should be, to put in context I wasn't driving physically or wasn't distracted from being on the road," he said

"It doesn't matter, it's still an offence and the simple rule is don't use your mobile phones for any purpose when you're behind the wheel, whether you are stationary or moving."

The Opposition said the incident showed the Government's driving warnings were not working.

"Troy admitted he made a mistake and paid the penalty. This should be the end of the matter and serve as a reminder to all of us about the dangers of using a mobile phone while driving," opposition police spokesman Guy Zangari said.