Hi Chucky,
Was Robert the youngest child? If not, then likely the births of his younger siblings would be registered after February 1856. 1 March 1856 saw the commencement of civil registrations in NSW, and the informant was required to supply lots of information that was not needed for noting on baptisms. The civil registration process required details about the baby's older siblings. So for example, the birth cert (or official transcription) for Robert's youngest sibling, if in the civil registration process, will have details about their parents (ages, places of birth, mum's maiden name, dad's occupation, when and where married) and about the older children of that marriage, AND for the baby whose birth is being registered, then there's also the info about where they were born, the name/signature of the person registering the birth and their relationship to that baby, and the names of the witnesses (including perhaps the midwife and the assistant).
If given the option, the official transcription is as good as the more expensive real deal certificate, and better still it has been transcribed by experienced people familiar with the handwriting of those deputy registrars from that era. So if you know the name of the youngest child, and they were born in NSW after Feb 1856, their NSW birth official transcription should give you a huge picture of the family unit for around $Au20.00
JM
