The 16-year-old broke his neck and suffered severe injuries when he fell six metres onto concrete in the accident at a block of units on October 27, 2016.

A subcontractor BCC Building had asked the student to attach wall fasteners and he had pulled up a stepladder adjacent to the void to complete the task, according to an agreed statement of facts tendered in court.

While he was on the ladder, an employee of BCC Building realised the student was in the wrong area and yelled at him "you're in the wrong f--king unit!"

The student descended the stepladder and stepped off the last rung and into the open and uncovered void, which was about 300mm from the ladder's feet.

The day before, the student and a friend had been instructed by BCC to carry plasterboard sheeting over scaffolding and across a void, using in part an aluminium plank. They struggled to carry the sheeting because of its size and weight.

"The wind was blowing gently but it was enough, according to the young men, to cause them concern and make carrying the sheet difficult and in their words 'scary'," the statement of facts said.

The developer Nikias Diamond Pty Ltd had contracted Feel Style Pty Ltd to complete all joinery work on the site and Feel Style contracted BCC Building Pty Ltd.

Nikias Diamond, the principal contractor, was charged over the breach and pleaded guilty in the ACT Industrial Court on Monday to an offence of failing to comply with its health and safety duty and exposing a person to a risk of death or serious injury.

The breach related to leaving voids open.

A defence barrister for the company told the court the students were part of the Kids Assist Program, run jointly with the ACT Education Directorate and the Master Builders Association.

Nikias Diamond was not participating in that program but the subcontractor BCC was. The barrister, Ian Neil SC, said there was no system where the subcontractor's participation in that program was communicated to the developer and it was unaware of the students on site.

Mr Neil said Nikias Diamond accepted the void was an obvious risk but that there were systems in place designed to avoid injury, including an exclusion zone delineated by scaffolding barricades and signage.

He said BCC contributed to the risk of injury when it circumvented the system and told the two young men to climb over the barricades and into the exclusion zone to complete work.

But prosecutor Sarah Beaumont said it was the construction company Nikias Diamond that had an obligation to maintain a safe work site, particularly in circumstances where there was no controlled access and it was "possible anyone could have wandered onto it with no experience and be at risk".

Ms Beaumont said there was a monumental failure in communication and the fault lay completely with the defendant Nikias Diamond.

She said that even if there was an exclusion zone, which was disputed, it was poorly signposted and the system in place was grossly inadequate.

Acting Chief Magistrate Glenn Theakston will hand down a sentence on September 9.