Simpson and Brown, with lead architect Richard Murphy Architects, acted as Conservation Architects to conserve and repair the auditorium at Perth Theatre, designed by William Alexander in 1900. It is an important regional theatre and a good example of a compact late Victorian auditorium design with good decorative scheme despite substantial repairs following a fire in 1924.

Simpson & Brown produced a Conservation Plan and Statement of Significance which informed the repair of the auditorium and justify the new theatre extension where it abutted the existing auditorium.

Works to the auditorium included refurbishment and repair of the existing seats, repair of existing decorative plaster finishes, replacing all non-original floor and wall finishes and upgrading the ventilation provision within the auditorium to new modern comfort levels, all in a sensitive manner.

A specialist decorative paint investigation of the auditorium was undertaken to understand the historical decoration schemes and a report produced. Little physical information of the original decoration survived the fire therefore, the historical interior was recreated according to historical descriptions of the 1920s scheme. Plaster specialists also faithfully repaired the ornate plasterwork where details had been lost