I am becoming confused, trying to follow this thread...
Either I have forgotten historical facts, or something is wrong somewheres....

1 July 1851 is surely the date for the commencement of Victoria as a separate colony, hived off from N.S. Wales .....
Some other confusions for me ....

Americans, and any person coming to mainland Australia could have reached any port by ship, the ships were not restricted to all arriving at the one port. Afterall, NSW was no longer a penal colony and transportation of convicts to NSW had effectively ceased in 1840.
Ships brought passengers to various ports around the coast and of course to several ports in Tasmania too.

NSW State Records Office has fantastic images of passenger lists free to search ONLINE .... and they definitely include free settlers into what is now Qld and Victoria

May I quote from NSW SRO's intro re that project ....
"This is part of a pilot project to digitise the following passenger lists: the microfilm copies of the List of Irish passengers arrived on the ship Sir Joseph Banks, Oct 1828 (NRS 5309); Persons on early migrant ships, 1828-32 (NRS 5310); Persons on early migrant ships, May 1832- Jan 1833 (NRS 5311); Women on the Red Rover and other early migrant ships, 1832 (NRS 5312); Persons on government ships, Aug 1837-40 (NRS 5313); Persons on bounty ships (Agent's Immigrant Lists), 1838-96 (NRS 5316); Persons on bounty ships arriving at Port Phillip, 1839-51 (NRS 5318); Germans on bounty ships, 1849-52 (NRS 5320); Members of the Family Colonization Loan Society, 1854-57 (NRS 5322) and Passenger lists of the Family Colonization Loan Society, 1854-55 (NRS 5323)." Here's the link to the resource at NSW SRO
http://www.records.nsw.gov.au/state-archives/guides-and-finding-aids/nrs-lists/nrs-5316Where you don't know the ship of arrival then perhaps it is best to first look that up via the keyword option under the passenger's surname .....
http://srwww.records.nsw.gov.au/indexsearch/keyname.aspx How could an American get to Melbourne in 1855? Via NSW, perhaps. He could also be one of the possibly thousands of migrants, who arrived having paid their own fares prior to 1852. Victoria has Assisted Migration lists for 1839 to 1871, (Index to Assisted British Immigration 1839-1871 ) and Unassisted migrants from 1852. Victoria was established as a self-governing colony in 1850.
NSW has Assisted migration lists to Port Phillip (Port Phillip, 1839-51 ). However, free settlers were not recorded, or records are not available, without intensive searching of ships’ lists.
Fingers crossed this helps
Adding : Oops I overlooked another link with passenger (and crew) lists .... again, it is an ongoing project, and it is of course looking for volunteers but the transcriptions are backed up with scans from NSW SRO reels
http://mariners.records.nsw.gov.au/Cheers, JM