"I was shocked and the look on his face will never leave me," Common told Metro.
"I parked the Land Rover behind the car, walked over and looked in the window. I saw the lad was naked and knew it wasn't right."
The farmer quickly called police, handing in Murray's car registration.
Murray told police the car window was steamed up from vaping.
However, police found pictures of the boy on Murray's phone.
Murray later admitted two counts of rape, two of sexual activity with a child, and four of causing or inciting a child to engage in sexual activity.
He was sentenced to 10 years in prison by a judge at Newcastle Crown Court on Monday.
The court heard that Murray had groomed the boy from the age of 12 and started sexually abusing him, reported the ChronicleLive.
The judge praised the farmer for his "resourcefulness, bravery and a public-spirited attitude" and awarded him NZ$760 as a token of gratitude.
"I'm really humbled and flattered but anyone would have done it. Nobody is going to put up with that," Common said.
After the case, Superintendent Andy Huddleston paid tribute to Common for taking action and helping police put a predatory paedophile behind bars.
He said: "Louis Murray was a predatory paedophile who abused his position of trust to befriend and groom a young boy. I am glad his behaviour has been uncovered and he is now facing a lengthy stint behind bars.
"If it wasn't for the actions of George Common that day, Murray's victim may have been subjected to abuse for a much longer period.
"Mr Common was quick to act when he thought someone was in trouble and did absolutely the right thing when he learned exactly what was going on.
"It was a brave act that has undoubtedly helped prevent a young person from being harmed further."
