It used to be the case, and still is in some churches, that notice of an impending marriage had to be read out in the parish church of both parties, on 3 successive Sundays, before the wedding.
See http://en.wikipedia.org/wiki/Banns_of_marriage
I think you will find that the wiki article is referring to the practices of the Church of England parishes in ENGLAND.
The practices of the Presbyterian Church in New South Wales AUSTRALIA were not ever governed by the Eccs laws of the Church of England. The civil laws of NSW from 1823 separated English Civil Laws (including the various Marriage Acts) from New South Wales Laws/General Orders of the Governor.
With the marriage banns just wondering what the difference is with a marriage certificate. It was in Australia, New South Wales and the Presbyterian Church in 1848.
It is not usual to find extant copies of Banns records for NSW. If the either the bride or the groom was serving a current sentence of transportation (not likely for a 1848 marriage) then permission to marry would need to be sought from the Prison authorities. Transportation to NSW effectively ceased in 1840. There were of course persons still serving their sentences in 1848 in NSW.
As the NSW BDM was not established until 1856, civil registrations don't commence until that time either. Prior to 1856 you need to rely on Early Church Records. The NSW BDM holds the authority to issue certified records of the ECRs. The ECRs are scant on vital information from a family history perspective.
There are some alternative ways to hopefully break down or at least make cracks in brickwalls.
Cheers, JM