Three of the six were suffering from suspected spinal injuries, police said after the Wednesday morning crash on the Bruce Highway, north of Brisbane.

Police said they arrived to find three of the vehicles on fire and the other four "extensively damaged".

The Abrams tank was due to take part in a major military exercise known as "Talisman Sabre", which starts on Saturday and will see 30,000 military personnel from Australia, the United States and other countries stage a series of live-fire drills.

Witness Naomi Wells told local broadcaster 9 News she saw a "massive explosion of fire" in the moments after the collision.

"As I was talking to the emergency people there were at least four or five explosions that had been going off and they thought at the time it might have been the gas bottles," from one of the caravans involved in the collison, she said.

The fire had spread to the surrounding grass area but Queensland Fire and Emergency Services said the flames had been brought under control.

Police said the cause of the crash was being investigated.