Safety concerns are hindering police investigations into a blaze that destroyed a Henderson plastics factory on Tuesday night.
Police and fire crews are investigating the cause of the fire and whether it was deliberately lit.
Henderson senior fire station officer Jim Maclean, who had been at the scene today , told the Herald police, insurance and fire investigators would return to the site on Thursday morning "looking at what needed to be done", but it was unlikely they would enter the charred remains of the factory.
"It will probably be days before people will be entering the building."
When it was safe to enter teams of investigators would "slowly and progressively" remove debris, he said.
Waitakere Detective Sergeant Ashley Matthews said officers and firefighters remained at the scene, but had not been able to enter the building because it was too unsafe to do so.
Machines may be needed to make the building safe and secure.
The investigation could take "some time" and the findings were unlikely to be released for several days, Matthews said.
A police spokeswoman said an officer was tonight guarding the site to ensure members of the public did not enter the building.
Fire crews left the scene around 6pm, a fire spokesman said.
The owner of the factory said he did not know what the blaze meant for the future of his business, but had urged his distraught staff to "stay strong".
Total Plastics owner Tony King said he did not know about the fire until this morning.
"I don't know what to say to be honest. I'm still numb."
He was insured but wasn't sure what the fire meant for his business.
He can't go into the severely damaged building for a couple of days, but was anxious to get access to moulds needed by customers, King said.
He arrived at the scene this morning, where he hugged some of his eight staff and offered words of comfort.
"They are very upset, understandably. I'm just making sure they get paid this morning. I've sent them home and told them to stay strong and I'll keep them posted on what the next stage is."
King did not know the cause of the fire but had been told it started in a back corner.
"There's nothing down there. It's strange, but that's for the fire investigator to find out what's gone on."
They had not previously had issues with building security, he said.
Waitemata Fire Area investigator Chris Lewis says the huge blaze, which lit up Auckland's night sky and destroyed two businesses, would have been less serious if the building had an alarm and sprinkler system.
"If they'd had a sprinkler system it would have kept it smaller until we got here and then we could've boosted it.
"It would've kept it to a smaller portion of the building."
Both systems cost to install and maintain, he said.
"But how much is this fire going to cost people? All this [burned material] has to go to the landfill, there's all this pollution that's run into the Oratia Stream, the building owner has lost a building and two tenants are going to lose customers.
"We always tell people with their business continuity plans, think of the worst thing that can happen.
"This is probably the worst thing that can happen."
Lewis and two detectives have yet to enter the blackened and mangled building formerly home to Total Plastics and Advanced Japanese Car Parts.
Henderson Fire Brigade senior station officer Jim Maclean expected to hand the building over to police this afternoon.
A dozen firefighters remained at the scene, Maclean said.
Pollution staff from Auckland Council checked Oratia Stream, which borders the building.
A walkway between Sunnyvale Railway Station and Aetna Pl remains closed.
Lewis said his work would include taking photos and looking at burn patterns on the outside of the building.
Investigators would also watch video footage of the fire, including clips posted online by witnesses.
He had spoken last night to the owner of the building, who leases it to the owners of the two destroyed businesses.
The owner, who he believed was insured, was distraught.
"He was a bit upset. Everybody's shocked when it happens to them."
The fire is being treated as suspicious.
A worker at the plastics factory said he was devastated.
Mike Hawthorne cut a forlorn figure outside Total Plastics this morning.
The 55-year-old plastics technician is afraid he may no longer have a job.
"It's devastating. I might not have a job and it's pretty hard to get one at my age."
He did not know what had caused the fire, but said they had previously had problems with "kids tagging the building and throwing rocks".
Robin Wright owns Coating Technologies, a paint supply business separated from Advance Japanese Car Parts by only a driveway.
He said the couple who owned the business had been at the site about five years.
"I feel sorry for them because they have worked bloody hard."
The fire burned for nine hours and its height involved about 100 firefighters and the city's entire aerial truck fleet.
There was still a smell of smoke in the air, but the scene was quiet and calm and people in the area had been told they can go about their daily business.
Pollution response staff from Auckland Council were this morning investigating a nearby stream for signs of toxic run-off from last night's fire.
A spokeswoman said results of any damage to the environment would take some time to determine.
The front of the Total Plastics building is still standing, but the rear of the building is a mangled mess of twisted structures surrounded by burned out cars stacked on top of each other.
Henderson senior station officer Chris Delfos said four crews remained on-site
The dense toxic smoke that poured out of the factory over surrounding suburbs no longer posed a risk to the public.
The fire broke out just after 10pm. Residents in the neighbourhood reported hearing explosions and seeing spectacular fireballs erupting 60m into the air.
The blaze posed a number of dangers for firefighters, including fallen powerlines, explosive debris, contaminated water runoff, dense smoke and unknown chemicals.
Nearby Henderson residents were advised to keep windows and doors shut when the fire broke out after fears toxic chemicals may have been in dense smoke plumes covering the suburb.
At least two trains were held up by the blaze after flames threatened the western train line behind the building.
Auckland Civil Defence monitored the scene overnight but said there was now no danger to nearby residents.