Lyn, Convicts and assisted passengers are listed by name in the Aus records. Unassisted passengers (i.e. those paying their own full fare) are not indexed and many are not recorded at all. Cabin passengers tend to be listed by name. Steerage passengers (the majority) are generally not listed by name. The ship's manifest usually reads simply "50 steerage passengers" for example. So, for unassisted passengers you have to search the actual ship's manifests, so it helps to have a very good idea of when the person arrived.
If you have a copy of the marriage certificate you refer to, it should have a column stating "how long in the colony". This should narrow the date of arrival for you.
I checked the 3 following indexes for you -
Assisted immigrants arriving Port Phillip 1839-1851 (Victoria
Assisted immigrants arriving Sydney & Newcastle 1844-1858 (NSW)
Assisted immigrants arriving Moreton Bay 1848-1859 (Queensland)
Alexander Galbraith is not on any of these.
CuriousDiana (Australia)