Fire Authorities believe a brake or wheel bearing malfunction is behind an "inferno" on a truck carrying dangerous chemicals.

The inferno closed the alternative State Highway 1 between Christchurch and Picton for nearly seven hours on Friday.

The road reopened about 2.30pm, but there were significant delays for a while after as built-up traffic cleared.

Matt Bryce A trailer fire on a remote part of SH7 closed the road for nearly seven hours on Friday.

Police said a truck and trailer carrying dangerous goods erupted into an "inferno" on SH7, 2 kilometres north of Engineers Camp, in the Lewis Pass, shortly after 8am on Friday.

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The driver was able to disconnect their truck from the trailer and drive away.

STACY SQUIRES/STUFF The Lewis Pass was closed for nearly seven hours while fire services battled a fire on a truck containing dangerous chemicals.

They escaped serious injury.

Hanmer Springs Volunteer Fire Brigade chief fire officer John Donaldson said it was thought the fire started due to a brake or wheel bearing malfunction.

Donaldson said the driver had seen smoke while driving, then braked to pull over. The trailer caught fire after they stopped.

STACY SQUIRES/STUFF As of 3.30pm, a fire engine, a tanker and two support vehicles remained on the scene.

He said there were 13 products in the trailer, ranging from "agricultural chemicals to cleaning products".

The fire was initially too dangerous for firefighters to battle due to the chemicals the truck carried.

"Some you can put water on, some you can't, so we had to sit watch as it burned down," Donaldson said.

STACY SQUIRES/STUFF There is a possibility the road will close again when the trailer is removed.

Fire and Emergency New Zealand spokesman Rewai Grace said a 3km exclusion zone was put in place and aircraft were not allowed to fly in the area. Nearby Boyle Village was evacuated.

Firefighters were eventually able to bring the blaze under control.

The burnt shell of the trailer remained on the side of the highway after the highway reopened.

Supplied The road closure caused a significant backup of trucks in Murchison.

Grace said there was a possibility the road would close again when the trailer was removed.

"They might be able to do it without shutting the road, we might have to shut the road, we'll just have to wait and see."

Grace said there was still a fire engine and a support vehicles from Hanmer Springs on site, which were expected to remain there until the trailer was removed.

Supplied Fire fighters had to let the trailer burn due to the type of chemicals it was carrying.

He said it had been a "very long day for those crews out there in these conditions".

"We couldn't have asked for a harder location."

Police Commercial Vehicle Safety Team (CVST) acting inspector Mike McRandle said the CVST would investigate.

STACY SQUIRES/STUFF Though the road has reopened, significant traffic delays are expected as built-up traffic clears the area.

Team members would talk to the driver and the company involved and review what the truck was carrying to make sure it was "appropriate". They would decide on the next course of action once they knew what had happened.

"Luckily and thankfully it happened in a non-built up area."

Engineers Camp resident Grant Nossiter said he and "a couple of village people" were given about 30 minutes to leave their properties after the blaze began.

"About 10 minutes before we left you could feel a strange burning smell on your tongue."

Power was out in the area and an emergency cell tower was installed, Nossiter said.

Boyle River Outdoor Education Centre manager Nick Chapman said police evacuated four staff as a "precautionary measure".

STACY SQUIRES/STUFF Fire services had the blaze largely under control by 11am, but the road did not reopen until about 2.30pm.

The route through the Lewis Pass became the main highway connecting Christchurch and Picton after the November earthquake badly damaged the coastal State Highway 1.

Snow forced the closure of the inland road several times this week, though the disruptions appeared to have had little impact on supply lines to the lower South Island.

Countdown spokeswoman Kate Porter said while there had been some delays around the country due to road closures, the company still managed to get trucks through once the roads reopened.

STACY SQUIRES/STUFF Fire services are yet to say the type of chemicals involved in the fire.

"We know when the weather gets cold our customers like to stock-up on all the essentials and our teams are doing a great job at keeping our shelves stocked."

Z Energy spokesman Jonathan Hill said there was "plenty of fuel in the South Island" so there had been no cause for customer concern.

BP spokeswoman Leigh Taylor said their stocks had not been impacted by the closures, and they did not anticipate any issues.

Supplied It's believed the fire was caused by a brake or wheel bearing malfunction.

"Our sites hold fuel in reserve which is further extended by the reduction seen in travel when bad weather hits."

The alternative route has been plagued by serious crashes and road closures since it became the main route.

Nossiter said the latest incident was "a bit more carnage and chaos for the damn roads, especially after the last few days with the snow".