A Queensland highway remains indefinitely closed as investigators seek reasons for the explosion of 50 tonnes of ammonium nitrate on a truck on Friday.

The scene of a massive truck explosion in outback Queensland has finally been made safe, more than two days after the blast injured eight people and badly damaged a highway and bridge.

Investigators will carry out a thorough inspection of the scene on Monday, after more than 50 tonnes of ammonium nitrate on a B-double truck exploded on Friday night.

Authorities say the blast, on the Mitchell Highway about 30km south of Charleville, resulted in a huge amount of damage.

The site was made safe on Sunday but rain meant investigators will get their first proper look at the scene on Monday.

“We’re talking up to two to three kilometres of damage to foliage, structures, the rail bridge, the bridge itself,” assistant fire commissioner Tom Dawson told the ABC.

Forensic engineers, mine safety officers and workplace health and safety officials are all expected at the site on Monday.

Police said the site was now safe, but the highway would remain indefinitely closed.

The 33-year-old driver of the truck is in hospital fighting for his life after suffering burns in the blast.

He had been transporting the ammonium nitrate – a highly explosive agent used in mining and farming – from Gladstone to South Australia.

The blast destroyed a bridge and injured a police officer and four firefighters called to the crash scene, as well as two motorists who stopped to help.

Two firefighters remain in hospital in Toowoomba in a stable condition, while two others were released from Charleville Hospital on Saturday with minor burns and cuts and bruises. The police officer who suffered cuts was also released from hospital on Saturday.

Two fire trucks were severely damaged, and shock waves rattled houses in Charleville 30km away.