Delwar Hossain, then 50, had on the morning of February 10, 2010, completed "lollipop man" work at Austral's Unity Grammar College and was returning equipment to a storeroom.

The blast erupted from an adjoining building's roof, causing severe burns to 30 per cent of his body and permanent hand and arm injury. He has since suffered depression and post-traumatic stress disorder.

Students were in class and none were injured.

Mr Hossain in 2018 sued the school and other parties involved in the college's construction for damages, arguing the injury ended his working life.

The NSW Supreme Court heard now-deregistered Binah Projects Pty Ltd was the main contractor in the construction.

It had used another now-deregistered firm, Enma Plumbing Pty Ltd, to install the college's LPG system.

The court found Enma Plumbing placed part of the gas installation in an unsafe position.

This caused an LPG gas leak to flow into the building's ceiling instead of outside, leading to the explosion.

The ignition for the blast was deemed to be the storeroom light switch Mr Hossain flicked on when he entered the room. He had the previous day noticed a "funny smell" in the room.

Justice Stephen Campbell on Tuesday awarded Mr Hossain, a Bangladeshi migrant, $3.12 million in damages based on loss of past and future economic loss, medical needs and physical and mental anguish.

The defendants will also pay his legal fees.

"He has suffered pain and suffering, loss of amenity of life, loss of enjoyment of life and the most significant disfigurement over the nine years and seven months since the explosion," Justice Campbell said.

"I regard his injuries as in a near catastrophic category."

Those damages will be covered by Unity Grammar College, Insurance Australia Limited as insurers of Binah Projects, plumbing and gasfitting contractor Five Star and gas tank and LPG supplier Elgas.

A cross-claim by the school against building certifier Cohen and Associates Pty Ltd, which failed to obtain evidence of the gas installation's compliance with safety standards, was also upheld.

Each party will provide 20 per cent of Mr Hossain's damages.

Five Star did not perform the defective installation but was involved in subsequent stages of the college's construction and failed to adequately inspect the installation or test for patent defects.

Elgas was found to have begun supplying gas to the school in February 2008 despite the absence of a "compliance plate" on the installation.

"I have found each of the responding parties liable in negligence ... In addition I have found Five Star and Elgas liable for breach of statutory duty," Justice Campbell said.

Australian Associated Press