John and Louisa's arrived to Australia prior to 1855, but when?

This research has been recently updated as of 29 March 2020 Click here for more details. The couple migrated to Australia on board the "Constance" in August 1854

John lived for 19 years in Victoria before he died after immigrating at the age of 23 in 1853/4[1]

Louisa died in 1901 after having lived in Victoria for 47 years which indicates that she must have also migrated in 1853 or 1854.[2]

John & Louisa married in Oct 1853 in England, so we know they were in England at this time. [3]

Their eldest son Thomas is born on 11 Jan 1855 in Waugh Terrace, Bourke Street, Melbourne, Victoria, Australia.[4]

They must have migrated shortly afterwards perhaps on the Mary Stoddart which sailed in April 1854 with only a small number of passengers including a Mr & Mrs Williamson (unfortunately first names and ages not listed).

This is the only possible match that I have been able to find  for the couple, the entry reads as follows:
WILLIAMSON ---- MR A
 WILLIAMSON ---- MRS A

It appears that they were travelling with no children and were both over 21. (Note the  “A” stands for adult not their first initial). [5]

Anon., 'SHIPPING INTELLIGENCE' The Argus, 3 April 1854, p.4 . Retrieved December 2, 2016, from http://nla.gov.au/nla.news-article4805280

There is no Louisa listed on the incoming passenger list at all in either the assisted or unassisted passenger lists for incoming shipping to Victoria in from 1853 to Jan 1855 time period.

Has anyone else found them? I'm desperately looking for help, any ideas of where to look next would be wonderful.

Links

To learn more about John & Louisa Williamson click here

Sources

[1] Death certificate of John Williamson, died 7 Jun 1874, Registrar of Birth, Death and Marriages, Victoria 6427/1874
[2] Death Certificate Louisa Williamson, died 6 Sep 1901, Registrar of Birth, Death and Marriages, Victoria, 9613/1901
[3] Marriage certificate of John Williamson & Louisa Walker married 10 October 1853, General Register Office, England, Dec quarter 1853, Shoreditch registration district, 1c/364/311
[4] Birth certificate of Thomas Williamson, born 11 Jan 1855, Registrar of Birth, Death and Marriages, Victoria, 218/1855
[5] Taken from the Index of Inward Passenger Lists for British and Foreign Ports 1852-1899, MARY STODDART B 066 001.