Witness Hayden Voice told the Herald he saw the explosion around 30 minutes after authorities were alerted to the fire.
He said he couldn't believe what was unfolding before his eyes.
"I was about 250m away and pulled over, when the flames went up I thought 'that can't be real. What am I looking at?'
"The building that caught fire is a two-storey building, and these flames were going up about four to five storeys high. It was insane."
St John's Ambulance said they took someone to Palmerston North Hospital with moderate injuries just after midday.
Six fire trucks were called to the scene.
The roof of the large iron corrugated shed collapsed and explosions were heard from inside.
A car inside the garage had also caught fire, according to police.
A man said he was at the site buying a ute when he heard yelling.
He ran to help put out the small fire, but says it reached an LPG tank and blew up in front of him.
The man believes a worker was using a tool near fuel that may have sparked the fire.
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