Crews from Kent Fire and Rescue Service worked through the night to tackle the blaze at Estuary View Business Park.
Stephanie Harris, who was in the area, said there was a "sudden" and "huge explosion" at about 22:00 BST on Friday.
An engineer who was repairing the gas main at the time is in hospital after suffering burns in the explosion.
Another man was treated at the scene of the blaze, which has now been extinguished.
Southern Gas Networks said the exact cause of the explosion has not yet been established.
It said 600 homes in the Old Thanet Way area were without gas and it was working to restore the supply.
A spokesman said: "It is too early to know what caused the fire last night but an investigation is under way.
"We would like to reassure the local community that we are continuing to closely monitor the situation to ensure their safety."
Kent Fire and Rescue said the gas main was isolated shortly before 03:00 on Saturday.
A spokesman said: "It is not known what caused the explosion at this stage."
A gas mains exploded into flames causing homes to "shake" in Whitstable, Kent.