A plumber was treated by St John Ambulance crews in Pinjarra on Monday after falling off a ladder into a trench, according to emergency services.

The man was treated at the scene.

A WorkSafe spokeswoman said the incident happened about 1.30pm on Monday in the Pinjarra Industrial Area near Munday Avenue and Yagan Street.

The spokeswoman said the man, believed to be in his 20s, fell into a trench while working on a water pipe.

WorkSafe will inbvestigate the incident.

A Department of Fire and Emergency Services spokeswoman said the hole was about seven metres deep.

St John Ambulance personnel said the RAC rescue heilcopter was called to take the man to hospital.