In the 19th Century, and throughout most of the 20th Century, in NSW, the surnames of babies born before their parents are formally married follow the same pattern as babies born after their parents are formally married, and so the family name for a baby is drawn from the surname of their mum at the time of their birth/baptism. If the baby's parents are married, then mum has usually taken on the surname of her husband. So it may be worthwhile looking through various indexes (NSW BDM, family search, commercial websites) under different surnames, or alternatively using the baby's given name and a wildcard option for the surname.
Cheers, JM
Susannah Gill from family talks was born 17th March 1842 in Woodville, NSW. We cannot find a record of her birth.
familysearch allows searching with a first name (
NO need to include a last name), and you can restrict the search further to a particular year, and of course to New South Wales, Australia.
https://familysearch.org/search NSW BDM online now allows you to select day/month/year to limit your search. Depending of if their Early Church Records include Susannah's baptism, you may be able to compile a list of 'Susannah' baptisms to then consider her family's likely surname.
http://www.bdm.nsw.gov.au/bdm_fh.htmlOf course, not all babies were baptised, and not all baptisms were for babies. Susannah may have been baptised several years later. If you have the baptisms for younger siblings, perhaps you need to search around those dates, in case two or more siblings were baptised at the one time.
Cheers, JM