Thanks for your suggestion to check the church registers. Frederick and Catherine were married by James Fullerton, a presbyterian minister, at Elizabeth Street Sydney. I googled the ministers name and found - "Fullerton was a somewhat controversial figure who was reputed to run a ‘marriage shop’ out of his home – many of his marriages were performed there rather than in his church. In 1851, he was tried in the Supreme Court for ‘illegal solemnization of marriage’. Fullerton was known not to ask too many questions, and on the registrations of marriages he officiated, the personal details are often scanty and incorrect."
I will now contact the archives office at the Uniting Church and see if there are any further details recorded for the marriage.