The 20.4m by 3.6m mural was found behind hardboard walls earlier this month during demolition work inside the building.
It is believed the mural was created for a Japanese-themed festival held in the hall in 1892.
Since the mural was discovered, there have been calls for the council to investigate whether it can be preserved.
An article in the Otago Daily Times on October 3, 1892 said the Garrison Hall was set up to represent a Japanese village, and included descriptions of the Island of Kinsu, a Sintu temple, trading junks, a Japanese house, an old feudal castle and the sacred mountain Fusi Yoma.
The fabric of the mural is now old and brittle. It is stuck to the wall with a multitude of small tacks and the paint has been described as water-based opaque.
Restoration or preservation would present many challenges.
Representatives from the Historic Places Trust, the Dunedin City Council, the Dunedin Public Art Gallery,and the Otago Settlers Museum met yesterday to discuss the future of the mural as work on digging underneath the hall continued.
Following the meeting, which was not open to the public, DCC acting property manager Rhonda Abercrombie said those attending had agreed to investigate the mural's significance.
It was also a brain-storming session aimed at finding possible ways to preserve the artwork, she said.
Discovery of the mural has created a resurgence of interest in Mr Robertson's artworks.
Port Chalmers Museum president Norman Ledgerwood said an album of Mr Robertson's paintings was donated to the Hocken Library last week, and staff at the Port Chalmers Museum recently discovered a painting titled Port Chalmers from Boiler Point by the artist.
The painting was donated to the Port Chalmers Borough Council in August 1952, and was hung in town clerk Bill Lloyd's office for many years.
When the boroughs were amalgamated in 1989, the painting was donated to the Port Chalmers Museum, where it has since been stored.
"He was a prolific painter, so it is possible there are still a few floating around Dunedin," he said.
