A WA man who pretended to be a police officer in order to stop children from climbing trees in a local park has been arrested.

Police say the incident happened in Secret Harbour about 10.50am on Sunday, May 19.

It will be alleged the man was wearing a black cap with 'POLICE' embroidered on the front, and a lanyard with 'SECURITY' written on the front.

Police said the man asked two children to stop climbing trees in the park, and told them he was a police officer.

The incident was reported to police and as a result of inquiries, a 77-year-old man from Golden Bay has been charged with one count of impersonating a public officer.

He will appear in court at a later date.