A series of elegant subterranean inverted arches designed by the man who was also the architect of First Church and Larnach Castle have, since 1882, been holding up the John Colours Ltd building in Crawford St.
The tenancy was something of a milestone for the precinct, as it reintroduced government offices to an area government departments previously inhabited.
Mr Forbes said under the building was reclaimed land, with only 3m down to the former sea shore.
The arches sit on a concrete footing, and span an area between blocks of Port Chalmers breccia that support large wooden piles.
The breccia blocks are large and hand-cut. They have diagonal side faces that wedge them into the radiating bricked arches.
The arches spread the load of the building bearing on the piles.
Mr Forbes said the design had held up well, though the building had sunk slightly at the edges because the footing was not quite big enough.
He said R.A. Lawson would have been experimenting with foundations as he was having trouble with structural issues at the Seacliff Lunatic Asylum, which he designed.
The problems at the asylum resulted in his fall from grace in New Zealand.
Heritage New Zealand regional archaeologist Matthew Schmidt said there were examples of such foundations in the United Kingdom at the time, so the idea might have been borrowed.
Dr Schmidt said documenting the foundations before they were covered up was important, as it was the only way information about such structures could be recorded.
''We're using it to build up a picture of Dunedin.''
Mr Forbes said Worksafe was expected to move in August.
He had recently got building consent to refurbish the building, work that would now escalate.
