It took more than 40 firefighters nearly two hours to control the blaze in a flat above the businesses in Waikaukau Rd, Gleneden, which police believe was deliberately lit.
The owner of one of the businesses, who did not want to be named, said police had told him a P lab explosion had caused the blaze.
Police had taken a white van away for testing, said the man, whose business was destroyed as the premises and its contents were extensively smoke damaged.
"This (P) scourge on our country is terrible," he told NZPA.
Police confirmed the clandestine lab team attended but said results were not available.
Witnesses saw a woman leaving the flat and driving off shortly after the fire started. Police were trying to find her and at least one other person believed to have been living there, Detective Sergeant Peter Litherland said.
Police did not know who the people were, and the property's owner had been unable to identify them.
"Obviously our focus will be to try and locate and speak with the occupants. I'm sure they have information for us but obviously they're not making themselves known to us at this particular time," Mr Litherland said.
The exact cause of the fire had not been established but all possible accidental causes had been eliminated, he said.
Senior fire safety officer Roy Warren told NZPA firefighters had initially feared there were people trapped inside the building when they arrived about 1.20am.
Crews wearing breathing apparatus entered the fiercely burning building, only to find it vacant.
This was increasingly becoming a problem for firefighters as more industrial buildings were converted into residential dwellings, he said.
"There's a high possibility now that when you get a fire at a commercial site that people are going to be inside. That just seems to be a way that Auckland is changing."
