A 17-year-old steel fabricator has fallen 12 metres to his death at the Old Post Office redevelopment site in the CBD.

St John Ambulance got an emergency call at 4.30am and took the boy as a Priority 1, with lights and sirens, to Royal Perth Hospital, but he could not be saved.

WorkSafe investigators are on site. Credit:Belinda Cameron/Radio 6PR

A WorkSafe spokeswoman said the boy was on night shift and was working on the installation of a glass ceiling in the building, which was being renovated for the new tenant, fashion retailer H&M.

WA Police coronial investigators and WorkSafe investigators were at the cordoned-off site on Forrest Place, near Wellington Street, early in the morning. They are likely to remain for at least the rest of the day.