An inmate has knocked a female staff member unconscious with a guitar at the Malmsbury Youth Justice Centre, only weeks after assaulting two staff using a similar weapon.

The woman assaulted late Wednesday afternoon was taken to Bendigo Hospital after the attack, but further details about her condition or the assault were not immediately available.

Malmsbury Youth Justice Centre Credit:Stefan Postles

The inmate was being housed in a unit in the secure area of the centre at the time of the attack, which is being investigated by police.

Work Safe and Community and Public Sector Union representatives are expected to inspect the site in central Victoria on Thursday to determine if staff can better be protected, and if urgent improvements need to be made, a spokesman said.