Dear Robyn & Sue,
Thank you for your suggestions.
Margaret Philomena Treanor is indeed Louis Hahn's wife but we can't find any record of her living with Louis after around 1891. She died at a sister's house in 1929 and was buried at Rookwood with her parents and sisters, no mention of Louis on the headstone.
We don't have any record of a second given name for Louis.
Louis Joseph Hahn (b. 1894) is a nephew of Louis Hahn (b ~1855).
We have checked the Louis Hahn who died in NZ in 1926 aged 74. He came originally from Lithuania so he's not our Louis. The hotel keeper Louis Hahn who is mentioned in various Westland NZ newspapers is related to the Louis from Lithuania, so once again he's not our Louis. From what I can piece together, this NZ hotel keeper's full name is in fact John William Louis Hahn.
Anyway we can't make any headway in finding Louis after 1891 which is why I thought shipping records might be useful, but I'm not very familiar with the NSW outbound records.
By the way, Louis' parents were Englebert (Engelbert) Hahn and Anna Aloisia Theresa Maria Kremer. They came originally from Eltville in the Rhineland.
Best regards,
CM