Actor Hugh Sheridan has taken to social media to detail a glassing attack he witnessed involving Hair castmate Matthew Manahan.

The Packed To The Rafters star said both his brother Zachary and Manahan were attacked after leaving His Majesty's Theatre in Perth.

Actors Hugh Sheridan and Matthew Manahan rehearse for the latest Australian production of Hair. Credit:John McRae

In an Instagram message posted in the early hours of Saturday morning, Sheridan said Manahan has been glassed "for no reason".

"Minutes later my beautiful brother was attacked by the same person just after leaving the theatre with me," he said.