The Kestrel Preservation Society is trying to get to the bottom of why the 111-year-old ferry sank at its Auckland berth six days ago.
The society has asked businesses in the area for any CCTV footage that might show the last hours the Kestrel was afloat at Wynyard Wharf.
The boat has since been partly lifted out of the water, but the hull remained on the seabed.
Kestrel Preservation Society member Hugh Gladwell said until the the hull was raised, the only hope of finding out what happened was from security footage.
He said the society did not think anything suspicious occurred, but it simply wanted to know why the boat sank.
"In some ways it is an academic inquiry but we are mystified as to why the bilge pumps didn't activate."
Mr Gladwell said it could be another three weeks before the hull could be lifted out of the water.