The blast happened at Salters Cartage, a hazardous materials recycling business, in an industrial site on Bolderwood Pl, Wiri, about 1.35pm.
The force of the explosion blew photos off walls a block away, shook buildings and was described as sounding "like a bomb".
Witnesses spoke of seeing flames leaping 30m into the air, and feeling buildings shake 5km from the blast site.
One man died in the explosion, Inspector Julia Lynch, area commander at Counties Manukau Police, confirmed this afternoon. No one else was badly injured, although St John said four people suffered minor injuries.
A number of the man's co-workers, who saw the incident, were being talked to.
"Obviously they're devastated, they're upset," Ms Lynch said.
"Police will be investigating the incident alongside WorkSafe New Zealand."
Salters Cartage managing director Ron Salter said the man who died was a contractor and was killed when a tank exploded.
Two contractors were on site, measuring some walkways, he said. One decided to carry out some welding, without permission, causing a tank to explode.
The blast blew out all the windows at Salters Cartage, Mr Salter said.
A worker at a nearby business said the explosion shook her building. Part of an exploding fuel tank had landed in a nearby yard, she said.
"It shook our whole house, [I] thought the windows were going to shatter, freaked the sh** out of me," Janelle Stubbington said.
Workers at neighbouring sites said they saw debris flying through the air and billowing black smoke.
"People were just running around and we could hear sirens," said one man who wished not to be named.
He worked at a transport company that backs on to Salters Cartage.
"It shook the building pretty badly and we saw something flying through the air, it came down and there was a puff of black smoke."
A large fragment of what is believed to be a fuel tank was propelled about 100m through the air and landed across the road from the scene of the explosion on the roof of a car.
Satnam Bains was working at the Clendon Wines store on Roscommon Rd when the explosion happened.
"Bottles on the shelf rattled, I thought there was an explosion outside on the road," she said. "A lot of people came out of their stores to see what had happened."
Graeme Cochrane, branch manager at Repco Manurewa on Weymouth Rd - 5km from Bolderwood Pl - said staff members felt the building and roof shake, and heard a thud beforehand.
Jason Farquhar told One News he saw a man standing on the roof of a petrol container.
He described the container as being like a silo and said the man appeared to be welding.
Mr Farquhar heard "a pretty big bang ... like a sonic boom" and saw a "shock wave" when the explosion occurred.
The person on top of the container was thrown in the air far away from the container, he said.
A worker at a nearby business told the New Zealand Herald a loud noise "like a bomb" was heard.
"It was a large bang noise and then the cops and ambulance and fire all came.
"It sounded like a bomb -- it was a big noise."
Terrance Matthews, who works at ABC Tissue on Roscommon Rd, said a set of roller doors about 12m by 8m shook violently.
"I thought someone had hit one of the columns, it was so violent, and we are like a kilometre away."
TransDiesel service adviser Peter Swann said his office building "shook like crazy".
"We thought it was a plane gone down."
A man who works around the corner from the factory also said the explosion shook his office.
"And massive pieces of debris were visible in the air immediately afterwards. There was a strong smell of gas in the area as well," he said.
Earlier, a St John Ambulance spokesman said four people received minor injuries in the blast, and another person was being assessed nearby for asthma-related symptoms.
The Fire Service said in a tweet the explosion was "huge".
Specialist fire investigators and dozens of emergency crews were sent to the scene following the blast. Roads around the site were closed for a time.
A WorkSafe NZ spokesperson said it had been notified about the incident and was investigating.
"WorkSafe is not making any further comment."
Salters Cartage is owned by Ronald and Natalie Thomas, according to public records.
It specialises in waste oil collection and recovery; shipping and marine collection and recovery; oil disposal; fuel and chemical delivery and hazardous liquid storage.
