The fire service says an explosion that destroyed the main warehouse at a car yard in Onehunga was caused by a welder working above surface oil and fuel.
The fire at Strong For Honda took 70 firefighters an hour and a half to bring under control. About 60 personnel were evacuated from the site on Wednesday morning.
Eastern Auckland chief fire officer Graham Fuller says there was plenty of flammable material at the site.
And fire service operations manager Roger Callister says it's lucky no-one was injured. He says many compressed air and fuel tanks exploded in the blaze.
Strong For Honda director Peter Bish says only about half of the employees worked in the warehouse itself. No injuries were reported.
He says workers will be able to keep their jobs.