The work - which will formally open next Thursday - has restored the main auditorium while also creating two new performance spaces.
Other alterations include work on public facilities such at the bars and lobby, and the backstage spaces.
The scheme also involved a budget for the provision of artwork as part of the redevelopment. Clem Crosby was commissioned to work on a series of painted panels on the facade of the main theatre, which are overlaid with expanded metal mesh.
Originally designed by Bill Howell at a cost of £60,000, the theatre building opened in 1970 and was only intended to be a temporary home. 30 years later, new work was judged essential and started on site in 2004.