In a letter to parents, principal Anna Wilson said the dust containing lead particles had been discovered during routine maintenance work in the school heating system during the summer break.

The dust was discovered in the boiler room and heating ducts near the school hall.

The highest reading was 8.89 milligrams per square metre (mg/sq m) which is eight times the acceptable threshold for inaccessible spaces (1.08 mg/sq m).

The heating system connects to the hall, library and five learning spaces.

Ms Wilson said the impacted spaces had been cleaned by specialists and licensed assessors had confirmed the spaces were safe to occupy.

"The affected heating system has been sealed and will not be used until it is safe to do so," she said.

"Specialist cleaners have started cleaning the ductwork and this work will continue out of school hours under the supervision of licensed assessors.

"I'd like to reassure families that the safety of our students, staff and the community is our highest priority."

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An Education Directorate spokesman said the presence of lead paint was known before the work was undertaken.

Three public schools discovered lead dust at their campuses in the latter part of 2020.

At the beginning of term three, 11 classrooms at Yarralumla Primary School were closed after lead dust was found after windows were replaced in the school holidays.

In September, five rooms were closed for cleaning at North Ainslie Primary School where lead levels 117 times the acceptable threshold were discovered in an art store room.

Lead dust was also discovered in ceiling and floor cavities at Alfred Deakin High School.

The Education Directorate estimated two-thirds of the ACT's public schools were built before 1992 when lead paint was commonly used in schools and housing.