Instead, it marked the second day of the inquest for the newborn, who died due to a catastrophic mix-up in a Sydney hospital.

He died less than an hour after delivery, when he was ventilated with nitrous oxide from a hospital gas outlet marked "oxygen".

"Already in a difficult week for John's family this will be an even more difficult day for that reason," counsel assisting the inquest, Donna Ward, acknowledged on Tuesday.

The court heard that John's distraught parents were too affected by the first day of the inquest to observe Tuesday's proceedings in the NSW Coroners Court.

Just how the laughing gas outlet in Bankstown-Lidcombe Hospital's Operating Theatre 8 came to be incorrectly labelled, and why the mistake was not discovered before John's death, were probed in the courtroom on Tuesday.

The piping bungle not only cost John Ghanem his life, but also caused severe brain damage in another newborn just weeks earlier.

Like John, Amelia Khan was ventilated with gas from the wall outlet shortly after being delivered in the same operating theatre.

Like John, she did not respond as expected to desperate efforts to resuscitate her. She was only revived when doctors removed the resuscitation machine from the oxygen panel in the wall.

She now has lasting and serious disabilities.

"A mistake was made, that mistake should have been picked up," engineering consultant Stuart Clifton, who investigated the incident for the local health district, told the court on Tuesday.

Connecting the wrong pipes happens often enough that installers should be careful.

But the fundamental issue was the failure to test the gas and piping when the new panels were commissioned, he said.

"The simple fact is that had they done the tests then ... this investigation would not have been required," he said.

Gas fitter Christopher Turner was last year fined $100,000 by the NSW District Court over John's death, after pleading guilty to workplace safety breaches.

Turner did not adequately check for cross-connection despite signing documents to say he'd tested his work, the court found.

An engineer employed by the hospital also signed off as a witness to Turner's testing, despite not having been present.

In the end, the piping mistake was only discovered a year later, in the days after John's death.

A paediatrician who'd treated Amelia read about an incident in India where a child had been mistakenly given nitrous oxide from a port labelled "oxygen".

He raised his concerns with a nurse, who asked for the gas panel to be tested. The hospital took six days to action her urgent request.

The tests revealed that no oxygen at all was coming from the oxygen port in the operating theatre.

Investigations revealed that the seeds for the fatal mistake were sewn some two decades earlier when the hospital was first built and the original pipework in the ceiling was mislabelled.

Turner and other technicians spent two evenings in July 2015 connecting those gas pipes to new panels in the hospital's surgical theatres.

Working in a cramped and dark ceiling space, they did not notice that the pipes were incorrectly labelled.

Unlike the usual set-up, the four pipes running parallel in the ceiling did not change direction equally, changing the order they were in.

That would have led to the "fundamental mistake" made 20 years earlier, said Mr Clifton.

But if the technicians had used the usual method for connecting pipes, the error probably would have been revealed, the court heard.

The inquest continues.

Australian Associated Press