Mr Yuile said the man sitting closest to the camping stove suffered burns to his face.
"They must have assumed it was safe. It was probably unlucky they didn't smell the gas."
He said the explosion, from a gas cylinder similar to a can of fly spray, may have prevented the three local men from passing out from the fumes in the enclosed cab "so perhaps they escaped serious injury".
Wairoa Volunteer Fire Brigade chief fire officer Grant Duley said they were called to the hospital to make sure the men's ute was not a fire risk.
He said there was evidence of seat covers having burned but there was no fire danger.
"It must have been like a flash and put itself out," he said.
"By the time we got there it was all out bar the shouting."
The men were flown by the Lowe Corporation Rescue Helicopter to Hawkes Bay Regional Hospital where a representative said they were in a stable condition and likely to be transferred to Hutt Hospital.