I believe that this child may be the son of an illicit affair between Ann Hardy and John Robinson (Convict) employed at one time in Public Works. The on-again and off-again affair between Ann and John is well documented in Carol Brills - Heaven and Hell Together. The following extract from Heaven and Hell Together provides what I believe is background for the unamed child, of which there were a number from the relationship between Ann and John.

There is circumstantial evidence that John Robinson was sent to the Jerusalem Probation Station, near Ann's sister Sarah. 30 Sarah and Mark Bunker's farm was just a few hundred metres down the road to Oatlands from the station, and Ann was recorded as living there a few years later, when she placed several of her children in the Orphan School. This conjecture ties together several facts: Ann gave birth to yet another son, Thomas, in late 1829 or early 1830. Thomas's birth wasn't registered, nor was he baptized, highly suggestive that he was John Robinson's son. A few months after Thomas was born, Ann admitted a "male ROBINSON" into the newly built orphanage at New Town, the first of all seven of Ann's sons to be sent there. This child's father was listed as "a convict in the Public Works" - but was, in all probability, Ann's oldest son John Hardy, William and Ann's first -born son. No discharge date was given for this boy, but children from the Orphan School were usually apprenticed out by the time they were fourteen years old or so.