The boat's owner, who was clearly upset, told a Bay of Plenty Times Weekend reporter at the scene he was inside waiting to pay for the petrol when the fire broke out.
The owner, who was from Taupo, raced outside and pulled his friend from the boat so service station attendants could hose him down to keep him cool until emergency services arrived.
He then moved the boat and unhooked it.
The man suffered serious burns to most of his body and was flown to Waikato Hospital in the rescue helicopter.
It took firefighters about 15 minutes to put out the fire.
Mr Beech said he was unsure how the fire started.
"It could have been electrical ignition or anything in the boat," he said.
"The boat's completely written off."
Underfloor tanks needed to be well ventilated to prevent such instances while filling up, especially in hot weather, Mr Beech said.
The Te Puke water tanker was brought in to provide water to douse the fire, he said.
Police and St John ambulance staff were also called to the scene.
St John Western Bay territory manager Ross Clarke confirmed a man in his 40s had been flown to Waikato Hospital with "serious burns" from the scene of the incident.
Gull general manager Dave Bodger said the service station had been closed and now had specialist contractors on site to determine the cause of the incident.
He was unsure when the station would reopen.
