I'm intrigued that Garnay does not appear in the archives d'outre-mer. There is a Garnier, Henri Paul Aristide
condamné au bagne en 1872 (Nouvelle-Calédonie)
who was apparently part of the Commune de Paris in 1871. Could this be your Garnay?
Are you sure that Garnay and Chappin in Sydney in 1885 and 1886 are the same person?
What else do you know about Henri Garnay?