Mr Johnston (21) got a "surprise" phone call from Murphy on Saturday to tell him he had won the Motor Trade Association's (MTA) automotive apprentice of the year award.
As part of his prize, Mr Johnston will travel to Melbourne to be alongside Murphy and his Castrol Racing pit crew team when they take to the Sandown Circuit racetrack for the Norton 360 Challenge in the V8 Supercar series on November 19.
"It was real shock to hear from him.
It's going to be awesome to be part of the whole atmosphere and be involved [behind the scenes] at the event," Mr Johnston said.
The Mt Aspiring Motors mechanic beat three other finalists, from Palmerston North, Auckland and the North Shore, to be the top automotive apprentice.
Mr Johnston missed last month's scheduled competition final in Wellington after the snowstorm which hit the lower South Island left him stranded when his flights from Queenstown, and then Invercargill, were both cancelled.
A rejigged final subsequently took place in Wellington last week and Mr Johnston said he had been confident throughout the "very close" competition.
MTA competition judge Jess Ross-Smith said Mr Johnston had shone in his interview.
"He was very personable and easy to understand. He has a clear passion for the motor industry and that was hard to ignore."
Mr Johnston also won a $2000 cash prize and a potential study grant from the Motor Industry Training Organisation.
He will travel to Nelson next month to receive his award from the MTA, accompanied by his mechanic mentors, Mt Aspiring Motors owners Dave Saunders and Riki Ryan.
