Fire and Emergency NZ northern shift manager Carren Larking said multiple calls were received about the fire since 1.06am.
"Some people said it was just a grass fire, some said it was a transformer on fire but on arrival we found 12-15 cars on fire in a car wrecker's yard."
Four fire trucks and crews were needed to extinguish the fire, on Great North Rd between Clark St and Rata St.
All had gone home before 6am, Larking said. A fire investigator would return to the scene at daylight to look at what caused the blaze.
A reporter at the scene said police had cordoned off the area, while another witness said police detectives attended.