Mr Morris said the offender or offenders managed to gain access into the locked yard, which was protected by a barbed wire fence, and may have used a nearby skip to scale the fence.
Some cars were tampered with at a neighbouring business property two weeks ago but otherwise the attack had completely come out of the blue, he said
"I don't know any reason why someone would have done this."
Detective Mark Farrell said police and a Tauranga fire investigator had launched an investigation but inquiries were ongoing.
How the vehicles were set alight was yet to be determined, he said.
Mr Farrell is urging anyone who saw any suspicious activity in the area early Saturday or knows anything about the arson to call police on 577 4300.