Primary Source Documents held by NSW Archives, and originated by the NSW Governors in the 1820s:
My transcriptions:
15 September 1825 Certificate of Freedom 054/4273 replaced CF 8/1807 for Mary PRESTON, Canada (2), 1810, ex Middlesex Gaol Delivery 20 Sept 1809, Seven Year sentence, native of London, a Housekeeper, (now aged) 34 years …… Grey hair …. See Reel 602 at NSW State Archives, as per the Convict index :
https://www.records.nsw.gov.au/archives/collections-and-research/guides-and-indexes/convicts/indexes Likely you would find digitised image of the official register of that replacement CF via Ancestry or FindMyPast as both have current partnership arrangements with NSW Archives. Likely you will find digitised image of the 1825 General Muster and of the 1828 NSW Census online at various family history websites.
1825 General Muster, Mary PRESTON, per Canada arriving 1811, Free by Servitude, Widow, McMahon, residing Windsor
NSW 1828 Census Mary PRESTON aged 38 per Canada 1810 at Pitt Town, Servant to James FLOYD.
I believe those official documents and the parish registers would be vital documents to consider as confirming/eliminating any Mary PRESTON as part of your family history, however, I do readily acknowledge that each family historian can decide for themselves how and what and when and where and why they want to include any person in their own records of their family trees.
JM