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Emergency services responded to the fire at the Wairau Diversion, north of Blenheim, about 8.30pm on Monday after noticing smoke during a training session at the nearby river.
At the same time, the couple - who want to remain anonymous - were metres away eating dinner with other campers when they noticed smoke coming from their vehicle.
"We ran over, but within 30 seconds it was on fire," the man said.
They thanked the Rarangi Volunteer Fire Brigade for trying to save their camper, "but it was too late ... We don't have much left".
The couple had been living in New Zealand since February last year, but moved into the self-contained minibus about five months ago. They converted the vehicle themselves.
The fire happened on their first night in the South Island at a freedom camping site run by the Marlborough District Council.
"[The] council will work with the individuals involved and insurers to undertake [the] removal of the van," she said.
Fire and Emergency New Zealand senior fire investigator John Foley said a firefighter would visit the vehicle to determine the cause of the fire. But the outcome depended on how much damage the vehicle had sustained.
Other diversion campers recounted their surprise at the fire.
"We didn't hear or smell anything that indicated it was on fire."
The pair tried to bucket water from the Wairau River to control the fire, but said by then the vehicle was already too far gone.
Others threw water on the grass to stop the fire spreading.
The German tourists the couple were having dinner with, who wanted to remain anonymous, said the flames spread quickly.
"It felt like it was over in the space of a couple of minutes."
"We popped our head over to see what had happened and saw the van was engulfed in flames ... It didn't burn very long. It was over in about five or 10 minutes, tops. It was scary."
A woman from near Auckland, who also wanted to remain anonymous, could not bring herself to look at the wreckage.
"I feel so terrible for them. You couldn't even smell smoke."
Marlborough woman Janelle White was collecting donations for the couple, asking if any locals could help the UK couple who had "lost everything". She said on Tuesday she had already received an "incredible response".
Police said they did not respond to the fire.
The Wairau Diversion was one of five freedom camping sites managed by the council. The council was considering improving the site's layout to enhance its health and safety.
The site could take up to 75 freedom camping vehicles.
To donate to the couple, call White on 027 618 7480.