Hi there,
NSW BDM Early Church Records includes four baptisms for William SIMPSON, son of William that may (and may not) refer to your chap. Not everyone was baptised, not all baptisms were for infants, not all baptisms were conducted at the same location as the birth place, and not all baptisms are included in the Early Church Records held by NSW BDM.
I can see FOUR possibles at NSW BDM online index:
1848 line 265, Vol 50, William, parents as William and Elizabeth
1848 Line 1466, Vol 145, William, parents as William and Rosanna
1849, line 1907, Vol 55, Wiliam, parents as William and Eliza
1849, line 1619 Vol 34A, William H, parents as William and Susannah (I assume this is the one you ordered, not sure why you consider this as appearing to be the wrong one)
I do think the ITM lookup request may help you further.
Then again, perhaps a request of the Church were the marriage took place? May be they can advise you where the Church register would be located, and if there is additional information on the register that is not on the NZ BDM issued marriage cert. I know that the NZ BDM print out is not just cheaper than the NZ BDM certificate, but often has a greater depth of detail recorded on it.
Re the NSW BDM online index does not give the district for these Early Baptismal Records, so there is no telling if any of the four certs I have noted above were for baptisms celebrated in the districts that became the colony of Victoria. But, the index itself does NOT date from the year of the event, in fact it dates from volunteers transcribing the records in the 1930s, so it may well be your chap’s possible baptism is one of the above.
At the FamilySearch website, in the Historical Recrods section, here’s two, there may well be others :
William Henry SIMPSON born 11 Feb 1849, Baptised 11 March 1849, at Gunning NSW parents as William and Susannah SIMPSON
https://familysearch.org/pal:/MM9.1.1/XTC2-5RX Gunning is of course located on the route to Victoria that was part of the overland way to Victoria in the 1840s (Hume Highway)
William SIMPSON, baptised 11 Feb 1849, AT Portland Head and Pitt Town, parents as William and Elizabeth
https://familysearch.org/pal:/MM9.1.1/XTDF-F8Q (Hawkesbury district, north west of Sydney)
https://familysearch.org/searchCheers, JM