The NSW upper house inquiry on Monday probed the presence of asbestos at Castle Hill High School, with members of the school community including teachers, parents and students invited to provide submissions.

The inquiry into the north-west Sydney school is taking place six years after the potentially lethal material was reportedly found on the grounds.

It comes after Castle Hill MP Ray Williams earlier this year told parliament that in 2016 the school community was told an asbestos test was negative when it was in fact positive - a hidden outcome that had put thousands of people at risk.

In his evidence at the inquiry, Mr Williams said he hoped the inquiry "ensured the future health safety and wellbeing of all students is not placed in jeopardy as I think it has been at one of my schools, that being Castle Hill High School".

But he defended former principal Vicki Brewer, saying he knew her for over a decade and viewed her as "outstanding" and respected by teachers and students.

"The academic achievements of that particular school speak volumes on behalf of her ability as a principal," he told the inquiry.

Earlier, teachers Shane Stubbs and John Connell spoke of their experience at the school, with Mr Connell testifying that classes continued despite asbestos being found in ceilings.

Mr Connell said unlike a previous asbestos incident at Beverly Hills Girls High School in 1993, which caused that school to be closed, a decision was made to remediate the situation at Castle Hill "and allow the school to keep running".

"There's nowhere to send the kids at Castle Hill High anymore, all the other surrounding schools are also full," he said.

Mr Connell testified "a whole bunch of directors were told about" the issue and said he "never thought of the concept that they'd actually done a test and it had come back positive, and then told us it was negative".

Pauline Hanson's One Nation MP Mark Latham questioned why the former principal had remained in charge "and how was the school still open".

Labor's Anthony D'Adam pointed to "systemic failures" and "fundamental cultural problems" at the school.

"(It's) around principals being incentivised to keep a wrap on issues that are brewing in schools," he said, calling schools a "black box" and principals the "choke point" on information flows.

Labor MP Courtney Houssos described the teachers' evidence as "shocking".

"I think that the scale of the cover up has been deeply, deeply disturbing and I thank you for your frank testimony," she said.

NSW Department of Education deputy secretary Leanne Nixon, in her evidence, said asbestos levels on site were now safe and stood by the actions of the department official tasked with investigating asbestos issues there.

"I just would go back to ... the accountable officer for a site around workplace health and safety is the principal," Ms Nixon said.

Speaking to reporters, Labor leader Chris Minns said it was "completely unacceptable" parents and teachers at the school were kept in the dark.

"You can't have a cover up when it comes to these important issues. You need to make sure this information is publicly brought forward and parents obviously have a right to know," Mr Minns said.

Australian Associated Press