Nelson police said residents in the area were asked to stay indoors, particularly those downwind of the fire in the Parkers Rd direction.
Police asked the public to stay away from the area.
A road block is in place as fire crew try to cool the cylinders down. Photo / Nelson Live
At this stage it was not known what had caused the fire.
Gym-goers were evacuated from City Fitness about 6.40am, with some reporting that they heard a "boom, boom" before people came in saying there was a fire.
The truck is thought to be a Summerland truck which had just arrived in Nelson from Christchurch carrying cylinders of oxygen, nitrous oxide and acetylene.