Witnesses have described explosions like “gunfire” and flames as high as 50 metres as a massive fire engulfed a chemical truck on Auckland’s Southern Motorway early today, forcing the closure of the motorway and evacuation of nearby homes.

Northbound lanes have just reopened after being closed for nearly seven hours, creating gridlock for commuters - but the fire has been extinguished after an hours-long battle by emergency services. About 15 homes and up to 30 residents were evacuated.

Mainfreight’s managing director Don Braid said the Chemcouriers truck was carrying cans of aerosol paint from Rotorua to Auckland. The driver had escaped unharmed and the cause of the explosion was under investigation.

Traffic has also slowed this morning around the Northbound Waterview Tunnel, which was temporarily closed due to a potential overheight vehicle. The tunnel soon reopened but Waka Kotahi warned motorists to expect delays.

Motorists coming from the North Shore into the CBD can also expect delays, as a breakdown - now cleared - blocked the third southbound lane on the Harbour Bridge.

Papakura resident Steven Shortt said he was woken by several explosions just before 3am, after the truck caught fire in a northbound lane, just north of the Papakura on-ramp - and near his house.

“Some of the explosions were massive... flying fire projectiles were getting thrown into our front lawn. Cars are covered in ash across the whole front. There were burning embers across our front lawn. It was pretty crazy, man.

The burning truck on the Southern Motorway. Photo / Steven Shortt of ShorttDrift

“I’ve never in my life seen anything like that before. The whole upstairs of the house is hot as against the windows. The explosions you could feel from across the road, the heat and the impact. It was like a war scene.”

Shortt said the rear carriage of the truck had been destroyed in about 10 minutes and the rest of the truck about 20 minutes later, amid repeated explosions.

 “With chemicals, we evacuated. We grabbed the dogs and animals, went across the road and watched the explosions and fires from a safer distance. Some of those explosions were well above power lines, 50-odd metres (high).”

Another resident told the Herald that the “fire was massive with ongoing, non-stop explosions for over half an hour”.

A Papakura resident living 3km from the blaze said she was awoken by the sounds of the fire shortly after 3am. “I thought it was gunfire,” she said. “The explosions were crazy.”

Firefighters at the scene of the massive truck fire on Auckland's Southern Motorway.

No one was injured. It is understood the truck driver ran from the fire and later made himself known to emergency services.

Mainfreight’s managing director Don Braid said the driver of the Chemcouriers truck which went up in flames on Auckland’s Southern Motorway this morning is shaken up but is fine.

”He attempted to disconnect the trailer but the trailer was alight by the time he managed to try and disconnect and had to move away from the vehicle as quickly as he could,” said Braid.

”He’s fine and a little bit shaken up.”

Braid said the truck fire is under an internal investigation.

”We think either mechanical or electrical (cause of fire) but we have to get that investigation completed before we can confirm one or another.”

Braid said the truck was carrying a full load of aerosol cans of paint from a client in Rotoura.

The load was due to be delivered in Auckland.

The fire had been contained by 6am but the motorway remains closed northbound between Papakura and Takanini and people have been warned to avoid the area.

Traffic has also slowed this morning around the Northbound Waterview Tunnel, which was temporarily closed due to a potential overheight vehicle. The tunnel soon reopened but Waka Kotahi warned motorists to expect delays.

Police said the truck was carrying “highly flammable gas in canisters and these have exploded over all of the northbound lanes and into the bush on the side of the road.

“The motorway is blocked but thankfully, no one was injured. Around 15 homes were evacuated as a precaution but have since been allowed to return home.

“The Southern Motorway remains closed northbound and is not expected to open for some time. There is a significant amount of debris to clean up off the road and the fire is still burning out.”

“There are diversions in place but as commuters will understand, this is going to cause significant delays and we would strongly advise people to avoid travelling this morning if they don’t need to.”

Up to 12 fire crews were at the scene at the height of the blaze. Fire and Emergency northern shift manager Carren Larking said six fire engines remained at the scene.

Kristina Pham was driving with her partner and children from Drury at 3am, making their way to the airport when they spotted thick black smoke coming from afar further down the motorway.

”Then we saw the flames and big explosions and when we got closer on the motorway - we saw the truck on fire,” she told the Herald.

”I didn’t want to drive past it as the fire was raging and so close and could explode right beside us. Cars in front of us, including trucks, were doing u-turns on the motorway so we followed them.”

The burning truck that has forced the evacuation of homes near a section of Auckland's Southern Motorway. Photo / Hayden Woodward

As Pham’s partner continued to drive away, sirens began to blare from all directions.

“[There were] fire trucks everywhere and police everywhere. It was unreal.”

Newstalk ZB sports broadcaster Andrew Alderson found himself trapped in gridlocked traffic in downtown Papakura following the chemical blaze.

”It’s mayhem at the moment,” he said, describing the “complete and utter gridlock” on south Auckland roads. ”All you can see are red tail lights in every direction, I’ve never seen downtown Papakura like this before.”

A distance away from where the truck fire sparked, Alderson said a dozen emergency service vehicles were still parked up on the southern motorway.

”It’s wall-to-wall gridlock, people are trying to snake their way through [the traffic], but good luck.”

A Shelter In Place notification has been issued for homes on Harbourside Drive, adjacent to the motorway, with residents being advised to remain inside their homes.