A Whanganui High School teacher has been charged with assaulting another staff member in a "serious incident" on Tuesday.

Police and ambulance were called to the school on Tuesday afternoon for the alleged incident between two staff members, Board of Trustees chairman Randal Southee said.

The man was arrested and would appear in the Whanganui District Court on assault charges, Senior Sergeant Andrew McDonald said.

Mr Southee said lessons had finished for the day when the incident occurred in one of the offices of the main administration building.

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"No students were involved and at no stage was their well-being compromised in any way whatsoever.

"This incident required police intervention and is currently subject to a police investigation as well as a Board of Trustees' investigation."

St John Whanganui territory manager John Stretton said paramedics were called at 3.17pm to assess a 60-year-old woman, but the patient had left by the time they arrived at 3.23pm.

Mr Southee said support was being provided for the staff involved.

"Should students feel the need for any support they can contact either one of our school counsellors through our student centre."

The school is working with police, the School Trustees' Association, and the Ministry of Education in relation to the incident.

Mr Southee would not comment on whether any staff had been suspended, as the matter is under investigation.