Explosions were heard coming from a warehouse that was consumed by flames tonight in the Auckland suburb of Penrose.

Emergency services rushed to the address on Great South Rd shortly before 9pm.

Huge flames were seen shooting from the smouldering building and thick smoke billowed into the air.

The road was cordoned off in both directions and vehicles turned away as sirens rang out into the night.

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Firefighters appear to have brought the blaze under control now but smoke is still rising from the charred wreckage.

Fire service area commander Richard Twomey said 12 appliances and 50 firefighters were currently at the building.

The building was understood to be unoccupied at the time the blaze broke out.

Twomey said fire crews were attacking the blaze from above as it's too dangerous to go inside.

Twomey said crews will aim to put the fire out within the next half hour but will stay on throughout the night.

It is not clear what started the fire but investigations are underway.

A witness at the scene said the building was completely engulfed in flames, and there had also been several small explosions.

"Fire trucks were trying to put the fire out with their aerial appliances - ladders and all that."

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Twomey said crews would remain at the building overnight as a safety precaution.

Eyewitness Vincent Vitolino said he was parked not far from the cordon when he saw the inferno.

"We saw police barricades and we just knew there was trouble.

"I can't believe how big it was. I thought it was just a small contained house fire but something as huge as this is really big."

He was not aware of anyone being hurt or trapped by the blaze.

Fire crew battle to contain flames taken hold of a warehouse in Great South Road Penrose. Video / Rosie Gordon

Another witness said the fire was in a warehouse that appeared to have been completely demolished.

A manager at GAS petrol station on Great South Road said the fire was just across the road from his own business.

"A lot of firefighters are trying to stop the fire but it looks like it's still going," he said earlier.

Photo / Rosie Gordon
Photo / Rosie Gordon

The man, named Raj, said it looked like the business up in flames was near Humes Pipelines Systems.

Train services on Auckland's Onehunga line have been suspended due to safety risks.

Auckland Transport spokesman Mark Hannan said the train lines were directly behind the factory, though it wasn't yet clear whether the flames had caused any damage.

"It's suspended until 10.30pm, which is the last train anyway."

Hannan said it was too early to know whether there would be any impact on train services on the line on Friday morning.

A mum who lives nearby said she was heading home from Greenlane McDonald's when her kids saw the fire trucks.

Louise Gould believed the building was the Humes building, which was about 100 metres from where she worked.

Her kids thought the fire was "pretty exciting".

"But it's not, it's actually quite terrible.

"We're waiting for the hoses to be cleared so we can move our car out.

"This is the first real tragic event they've seen."

A crowd of people watched on as fire crews fought to bring the blaze under control.

Workers wearing Humes labeled jackets appeared teary-eyed as the gathered outside the burning building.

A spokeswoman for St John ambulance said crews attended but this was simply to support fire crews.

"There were no patients to assist," she said.