The NSW Government has announced a full rebuild of the Broken HIll high school after an independent hygienist found the mould outbreak could pose future health risks.

The mould was discovered at the end of the 2023-2024 summer holidays before the school was due to reopen.

"The mould outbreak was likely caused by a combination of factors including the moisture from the school's air conditioning which was left running, end-of-year carpet cleaning, Broken Hill experiencing an unseasonably wet summer, and storage issues impacting routine cleaning," the NSW deputy premier's office said in a statement.

Multiple types of mould were found in the school including Aspergillus and Penicillium spp. These strains are dangerous human pathogens.

The mould would be difficult to entirely remove from the building and could reappear, the independent occupational hygienist's report stated.

The hygienist told the NSW Government that the school's mould outbreak was a Condition 3, the most severe classification.

"While today's news is not ideal, I want to assure the people of Broken Hill that we will be with you for as long as it takes to rebuild this school," NSW deputy premier and minister for education Prue Car said.

"I hope that those involved in the consultation can use the process to tell us about the facilities they need at at the new Willyama High School, to set Broken Hill up for future success."

Willyama High will be rebuilt on the same site. In the meantime, students and staff have been spread across three other Broken Hill schools.

"The students will reunite at a pop-up school Willyama High School on the grounds of Broken Hill High School from the start of Term 2 on April 29," the deputy premier's office said.

"The pop-up school will provide students with high-quality facilities and materials, ensuring Willyama retains its own identity while the school is rebuilt."

Mould in classrooms could be a serious health risk to students and teachers, causing headaches, nausea and respiratory issues, Melbourne University professor Rebecca Bentley said.

No level of exposure to mould is considered safe, according to the World Health Organisation.

"It's not surprising when you get humidity that's as high as it has been in some parts of the country that you're gonna start to see more mould growth in buildings," Professor Bentley said.

NSW Teacher Federation deputy president Amber Flow said "there are 2200 public schools, so of course, some of them are significantly aged".

But no NSW public school, other than Willyama High, was found with mould after the school holidays, she said.

"I think it's fair to say that in Broken Hill they have particular weather complications that we don't see in other parts of the state in terms of the heat and the humidity," she said.

Ms Flow said cleaners and other employees at the school were expected to notify the Department of Education if mould growths were spotted.

People working on site at Willyama High School during the holidays, as well as cleaning staff, flagged the mould to the Department of Education, Ms Flow said.

An independent hygienist examined the school and warned it was not safe for students and teachers to return to the facility.

But mould is not always visible, Professor Bentley said.

It can be under flooring, behind furniture or walls.

"But you might smell it. You might walk into a room and you might think there's sort of that damp musty smell," she said.

Professor Bentley emphasised the need for classrooms to have good ventilation to avoid mould-related health issues.