Well this is certainly a "solid" brick wall that I have hit!!
Received emails back from the Librarian services of the Presbyterian Church archives, who very nicely told me that they did not hold the marriage records for Eulomogo Presbyterian Church. He directed me the Anglican Archives in Parramatta, who he said would hold the records for that period.
Today received a warm email back from the Anglican Archives in Parramatta, saying that the earliest marriage record for the Presbyterian church in Dubbo area is 1877. Elizabeth Graham was married in 1874!! They also kindly said that "We do not have any references that suggest an earlier Marriage Register for Dubbo Presbyterian Church area is held by another agency."
It was so easy to ask these services for help and they were more than willing to suggest an alternate source if needed.
So, it's back to square one for me!!