I hope I am not straying too far off track but ...
The NSW BDM’s records include the following marriage
James STEPHENSON of the Parish of Dungog, a Widower, and Sarah Annn PITT, of The Allyn River, Spinster, married by License by Arthur Wayn, with the Consent of the Parents. The Witnesses were John Anerback of Altona, Hamburg and Hewitt Tate of Dungog.
The above info is from the NSW BDM image that has been uploaded to a commercial website. The document was certified on 10 Nov 2004. It clearly has PITT not FITT. It is referenced 1111 Vol 43B.
See later post, marriage is 1856, not 1855. I think James and Sarah Ann may have been in significant trouble with the police over the death of one of their children. If you read the cutting carefully, you will find mention of Sarah Ann, her husband, three children, including a child of seven years and the grandfather of the children too. You can see that James (husband of Sarah) had died after the death of Charlotte and before the court case commenced.
https://trove.nla.gov.au/newspaper/article/18701478 Maitland Mercury 11 April 1865. I wonder about the Grandfather mentioned in this cutting … Has he been identified by name anywhere? What was his surname? Charlotte's NSW birth cert should provide info about all her older siblings, and about her parents ... including of course where and when James and Sarah Ann were born... that info would help validate info already known to our OP.
The NSW BDM online index has the death for James STEPHENSON, aged 67 years, died Dungog. Reference 3583/1865. The index does not display his parents given names suggesting to me that perhaps … ‘Grandfather’ was not related to James STEPHENSON but to Sarah Ann FITT…
December 1868, Sheriff’s advertisement of a Sale of five allotments in Dungog, one with a brick cottage and outhouses, Sarah Ann as Admon …
https://trove.nla.gov.au/newspaper/article/13177127 SMH 10 Dec 1868
December 1872 I think Grandfather was Robert FITT …. Sarah Ann Stephenson and Mrs Stephenson mentioned in the article … at Gostwyck, Upper Paterson
https://trove.nla.gov.au/newspaper/article/18769726 Maitland Merc 31 Dec 1872
I have not found a likely death for a Robert FITT. The d.c. if in NSW, VIC or QLD should list the names of his offspring and his origins. That family history info would be useful to our OP to help with further DNA findings…
ADD
This is from 1863. Stolen from Wire Gully, near Dungog, …. Reward … restoration of the horses to James FITZSIMMONS, Wire Gully, near Dungog … so I wonder how this James FITZSIMMONS is related to our OPs Benjamin FITZSIMMONS of Dungog. I wonder if this James FITZSIMMONS is the husband of Mrs FITZSIMMONS of Wire Gully in the 1875 PO Directory.
https://trove.nla.gov.au/newspaper/article/18702416 Maitland merc 18 June 1863 ….
Question if I may ... what dates for the embezzlement action brought by James STEPHENSON against Benjamin FITZSIMMONS ... or dates for when Sarah Ann STEPHENSON was in Maitland Gaol at same time as Benjamin Fitzsimmons ...
I have his arrival papers and ticket of leave (attachments too large). The arrival can be found online at the Australian Biographical database and other sources, including: https://convictrecords.com.au/convicts/fitzsimmons/bernard/132556. In NSW BDM he is registered twice, as Stephenson and Fitt, both with the same number (10234/1869). My mum's uncle Godfrey Stephenson said George Stephenson's nickname was "Fitzy". There are many coincidences, such as he and Sarah serving time together in Maitland Jail and the fact that Fitzsimmons was charged with embezzling Sarah's husband James Stephenson. No, I can't find a record of Louisa's death either but the birth is 8900/1867. I do not know if he was in Victoria.
Thanks,
JM