Cromwell volunteer fire brigade chief fire officer Steve Shaw told the Otago Daily Times this afternoon it was believed there had been a "transfer of heat from the exhaust system into an area of garden covered in bark chips".
"It's really dry at the moment and those vehicles run really hot. If it was any other time of year it wouldn't have happened."
Nearly 50 tourists lost their possessions after flames rendered the bus a ''skeleton'' near Cromwell on Wednesday afternoon.
None of the 48 passengers on the Trafalgar bus destined for Queenstown were harmed in the incident.
The Travel Corporation head of public relations Vanessa Budah said the coach was chartered from a third party, Pacific Tourways, which was investigating the cause of the fire.
The bus was carrying Trafalgar and AAT Kings tourists.
The companies are owned by The Travel Corporation.
Cheryl MacKenzie, who works at the stall, said the fire started while the vehicle was parked and passengers were shopping.
''The bus is pretty much burned right out.
''There were lot of people in it, and all their luggage is gone. They've all lost their passports, everything.
''There's just a skeleton of a bus.''
Tourist Luke Grennon, from Ireland, said he lost most of his valuable possessions in the fire, including his suitcase, clothes, iPad and a camera lens.
''We were in the shop and we came out and heard a bus was on fire.
''We saw strong black smoke and flames just spread. It started from the back of the bus.
''I'm happy no-one was on the bus. It's so unfortunate, but luckily everything can be replaced.''
Tourist Roberta Ward, of California, said the bus had transported the group around the South Island.
It had travelled from Franz Josef yesterday.
''I was buying an ice-cream and we saw the black smoke come out. We had all our belongings, medication, jewellery, passports.
''I'm concerned about how safe buses are. Ordinary buses don't blow up like that.''
The group left to travel to Queenstown on a Scenic New Zealand Queenstown bus at 4.50pm.
Staff at the stall gave bus occupants free food and ice-cream while they were waiting for their replacement transport to arrive.
Owner Eletheria Jones said she had never seen anything like it in the 37 years she had run the stall.
Fire and Emergency New Zealand communications shift manager Tim Reynolds said vehicles from Cromwell, Alexandra and Clyde were called to the scene about 3.20pm.
It took more than an hour to extinguish the blaze, he said.
