When he was in Australia, Mark AMOS was not recorded as father on any births in Victoria with Rosa Louisa as mother.
There were at least two with Eliza TREMAIN after the 1865 marriage.
Because of the distances of continental Australia and the enormous serge in population in the 1800's (including convict transportees), there was a "rule" that a new marriage could be made if one had not had contact with a former spouse for seven years. It could be assumed that the former partner was dead.
Many a new marriage was made on this basis without need for divorce and if undertaken in good faith, was not considered bigamous.
ADDING
.I believe she went to Australia with him later that year he was prospecting for gold.He went on to marry Eliza Sarah Tremain in Australia in 1865 in Melbourne.
Both Rosa and Mark AMOS can be seen on Assisted Immigration lists and their limited details are available if required.
He was an engineer and there are many references to him in the newspapers of the time in the district of Fryer's Town, a gold mining area in the 1850's and 60's.
Sue