The original building was designed in a Gothic Revival style with Early English influences popular in England in the 1940s and reflecting English Catholic Liturgical practice, particularly evident in the work of renowned English architect Augustus Pugin (1812 - 1852).
The Cathedral was constructed in red brick with decorative stone coursing and mouldings from local Mount Lambie sandstone.
Internally the Cathedral consists of an aisled nave with round sandstone columns and Gothic arches.
The timber hammer-beamed trusses spring from corbels high on the walls to support the high open roof.
The aisles are low with a high level celestry and the Sanctuary and Chancel at the western end were substantially extended in the 1890s and 1960s.
There are a number of fine stained glass windows from various periods.
A high Nave and stained glass window above the gallery by John Hardman and Co dominate the western end.
The brick and stone crenelated tower marks the original west end prior to the addition in the 1960s of a Narthex.
The high spire proposed in the original drawings has yet to be completed.
The exterior includes a number of finely carved stone mouldings, decorative bosses, gargoyles and carved heads to windows and doorways.
After Vatican II some minor alterations also occurred to the sanctuary, altar and bishop's seat.
The Cathedral was constructed in five principal phases as follows: Cathedral during 1857 61, Chancel and North Trancept in 1897, Blessed Sacrament Chapel in 1897, Sacristy in 1922, major extensions in 1962, and Altar modification in 1981.
Subsequent modifications continued to use this style and adopt the same materials used for the original.
Substantial extensions and modifications undertaken in 1961-62 were to commemorate its Centenary and were designed in the Post-War Ecclesiastical style to complement the original design.

The former St Mary's Primary School is a two-storey building built in a U shape, the front block containing 4 classrooms, a central hallway and stair and two parallel wings at the western end of the central hallway.
Constructed in reinforced concrete the exterior is rendered in rusticated stucco with several architectural elements rendered in smooth stucco.

Elements of the front block are remnants of the original residence built on the site.
These include the front brick walls the northern brick wall.
elements of the southern brick wall and some sub floor elements including footings
The cathedral is generally in excellent condition structurally but there has been extensive deterioration of the relatively soft Mount Lambie sandstone used externally.
There is also evidence of rising damp and salt attack, particularly in the south-west corner of the building with some inappropriate attempts at repairs in the past.
Some of the damp issues relate to subfloor ventilation issues resulting from rising ground levels but also the use of later concrete slabs in later modifications.
The former St Mary's Convent, demolished in the late 1970s, lies immediately to the south of the Cathedral and the site of the original 1840s Catholic Church of Saint Michael, demolished in the 1870s, lies to the west of the Cathedral.

The cathedral complex has a reasonably high degree of integrity despite the loss of the adjoining Convent in the 1970s and original Church in the 1870s.
The 1880s Presbytery, on the corner of Keppel and George Streets now the Chancery Office, the 1930s Catholic is also an important element.
The original building was designed in a Gothic Revival style with Early English influences popular in England in the 1940s and reflecting English Catholic Liturgical practice, particularly evident in the work of renowned English architect Augustus Pugin (1812 - 1852). The Cathedral was constructed in red brick with decorative stone coursing and mouldings from local Mount Lambie sandstone. Internally the Cathedral consists of an aisled nave with round sandstone columns and Gothic arches. The timber hammer-beamed trusses spring from corbels high on the walls to support the high open roof. The aisles are low with a high level celestry and the Sanctuary and Chancel at the western end were substantially extended in the 1890s and 1960s. There are a number of fine stained glass windows from various periods. A high Nave and stained glass window above the gallery by John Hardman and Co dominate the western end. The brick and stone crenelated tower marks the original west end prior to the addition in the 1960s of a Narthex. The high spire proposed in the original drawings has yet to be completed. The exterior includes a number of finely carved stone mouldings, decorative bosses, gargoyles and carved heads to windows and doorways. After Vatican II some minor alterations also occurred to the sanctuary, altar and bishop's seat. The Cathedral was constructed in five principal phases as follows: Cathedral during 1857 61, Chancel and North Trancept in 1897, Blessed Sacrament Chapel in 1897, Sacristy in 1922, major extensions in 1962, and Altar modification in 1981. Subsequent modifications continued to use this style and adopt the same materials used for the original. Substantial extensions and modifications undertaken in 1961-62 were to commemorate its Centenary and were designed in the Post-War Ecclesiastical style to complement the original design.
The former St Mary's Primary School is a two-storey building built in a U shape, the front block containing 4 classrooms, a central hallway and stair and two parallel wings at the western end of the central hallway. Constructed in reinforced concrete the exterior is rendered in rusticated stucco with several architectural elements rendered in smooth stucco.
Elements of the front block are remnants of the original residence built on the site. These include the front brick walls the northern brick wall. elements of the southern brick wall and some sub floor elements including footings
The cathedral is generally in excellent condition structurally but there has been extensive deterioration of the relatively soft Mount Lambie sandstone used externally. There is also evidence of rising damp and salt attack, particularly in the south-west corner of the building with some inappropriate attempts at repairs in the past. Some of the damp issues relate to subfloor ventilation issues resulting from rising ground levels but also the use of later concrete slabs in later modifications. The former St Mary's Convent, demolished in the late 1970s, lies immediately to the south of the Cathedral and the site of the original 1840s Catholic Church of Saint Michael, demolished in the 1870s, lies to the west of the Cathedral.
The cathedral complex has a reasonably high degree of integrity despite the loss of the adjoining Convent in the 1970s and original Church in the 1870s. The 1880s Presbytery, on the corner of Keppel and George Streets now the Chancery Office, the 1930s Catholic is also an important element.
Cathedral of St Michael and St John