Dillon Wu was in his second week of a welding apprenticeship when he was asked to clean a large metal tanker alone for the first time.
But when he did not come out for a smoko break his colleagues realised something was wrong.
The 20-year-old died inside the tank, suffocating from leaked argon gas.
Marshall Lethlean Industries was on Friday fined $600,000 over the worker's October 2018 death, for failing to provide a safe workplace.
Mr Wu was employed by Australian Industries Group and was completing the practical components of his apprenticeship at Marshall Lethlean.
The company had moved to a new site and had not yet fully implemented a working system that ensured its workers were safe, the Victoria's County Court heard.
The day before Mr Wu got inside the tanker to clean it, a defective wire feeder leaking argon gas had been left inside.
This meant gas was flowing inside the tank, reducing the amount of oxygen.
Mr Wu climbed inside the tanker using a ladder but did not come out when the smoko break bells rang.
Other workers noticed he was missing and pulled him out. They performed CPR but he died at the scene.
Mr Wu's father said his son's image often appeared in his head, causing him to make mistakes at work.
Judge Douglas Trapnell found the company had no system to routinely inspect or maintain welding equipment and did not ask workers to turn off the argon gas main at the end of use.
"I find the company took no steps to insure against the risk of injury or death associated with a gas leak from defective welding equipment," he said.
"In my opinion, that conduct amounts to evident disregard by the company for the safety of Mr Wu and others at its workplace."