Children as young as 8 had their burns doused in water at the scene, a nearby restaurant owner said. He said he helped a severely burned child.
The man described wrapping a child in food wrap and pouring cold water on them - something he said he would never want to do again.
Mr Hall said firefighters were called about 2.50pm to the blaze, which was "well involved" by the time they arrived.
Fire and Emergency went to "third alarm" and had six appliances with 30 crew members.
Mr Hall said it was in a derelict building not in use for anything and there were no concerns about the building or any hazardous material.
"We managed to get the fire under control quite quickly but unfortunately there were some casualties."
Smoke was seen billowing from a commercial address on Heretaunga St West, where police closed the road.
Staff from the next-door building, David Jones Motors, were out dealing with a car when they saw flames rise up above the neighbouring fence line.
One staff member rang 111 while others, including a volunteer fireman, grabbed hoses from their garage and started spraying over the fence.
A nearby business owner said he could see a lot of smoke coming from the fire. He had heard a young boy had been burned.
Motorists were asked to avoid the area and traffic was diverted away from the Maraekakaho Rd and Stortford St intersections. Diversions had been in places at Townshend St and Heretaunga St West.
Police helped with traffic management because of the thick smoke that was in the area.
It was earlier believed the fire was at the Storty Bar on Heretaunga St West. The bar was closed after a fire in early last year.
Police would like to hear from anyone who witnessed unusual activity near the property prior to the fire. Information should be provided to Hastings Police Station on 06 873 0500 or alternatively anonymously via Crimestoppers on 0800 555 111.
