Fire Service northern communications manager Colin Underdown said at the time the building was engulfed when the first crews arrived.
It took firefighters more than an hour and a half to contain the inferno.
The fire also destroyed 14 vehicles inside, 10 of which were owned by customers of the business.
The final cost of the fire was estimated at $1 million, the court heard.
Fire investigators later determined it was arson, before Walker and Turner were linked to the blaze and arrested by police.
Justice Toogood said Walker had been recruited to burn the business and then hired Turner to help him.
Both pleaded guilty, but Walker believes he was "set up to take the fall".
Crown prosecutor Scott McColgan said the arson displayed signs of "significant premeditation" and was a risk to other businesses nearby and the firefighters who battled the flames.
Justice Toogood agreed.
"This was a major blaze and it did impose risks to those whose duty it was to put out the fire," he said.
However, the judge added that the timing and location of the arson attack "reduced the likelihood of the harm to the public".
Walker, a gang associate, was on bail at the time of the arson.
Turner, who was sentenced to two years and four months in jail by Justice Geoffrey Venning last April, said he was heavily addicted to meth and agreed to commit the arson to fund his addiction.
He said he was offered $5000 to burn down the premises.
The owner of the mechanic's shop said he has lost most of his business since the fire, causing a great deal of stress to him and his family.