The project, which it says is the largest single spend in the history of the archdiocese, includes two three-storey office buildings as a headquarters for Catholic services, a 44-unit age care unit complex, a meeting hall, an underground car park and a new presbytery.

Archdiocese financial administrator Helen Delahunty said while the project's deadline had been pushed back from December 2016 to April 2017, the project was going smoothly.

"It has just been pushed out a little bit. There was quite a lot of work done before we could even demolish that has put us back a few months, but it should take two years," she said.

Ms Delahunty said it was hoped building work would begin next month.

"The demolition has taken over six weeks, and we are nearing the end of that, and so we are hoping by the end of June everything will be down and then the building process will start with a great big hole in the ground," she said

Archbishop Christopher Prowse would conduct a sod turning ceremony on June 26, Ms Delahunty said.

Most of the Haydon Centre, a parish hall, was demolished in May, and the former Catholic Care building was pulled down in June.

The demolition of the cathedral presbytery, which housed priests until early 2015, was under way.

A small part of the Haydon centre was still standing while the archdiocese dealt with some electrical issues, Ms Delahunty said.

Archbishop Prowse announced the new administrative centre would be named the Thomas V. Cahill Centre after Archbishop Thomas Vincent Cahill, who served as archbishop from 1967 until his death in 1978.

The independent living units will be called Marian Place.

The church hopes to gain finance for the development by renting out church property in Braddon and from revenue from the on-site units.

The project will be managed by Joe Mammoliti from Hindmarsh Construction.