Thanks for your reply Riley,
I have the birth registration entries for three of Richard and Mary's children (including my Great-Grandfather) who were born in the Victorian goldfields 1864 onwards.
On each of these the place of marriage is listed as Dunedin, New Zealand, and the year of marriage is shown as 1861.
I have confirmed Mary's baptism date as 20/11/1843, so she would obviously have been no older than 18 at the time of the marriage. Richard would have been about 33 or 34.
Since you have been unable to find a reference to their marriage, I am beginning to wonder if they were married at all - it would not be outrageous to assume that they 'eloped' to Australia and continued to live as man and wife.
Richard may well have been a bit of a sly old dog, as was (or so it seems) my Great-Grandfather. I'm pleased to say it's not a family trait I have inherited (happily married, children born on the right side of the blanket).

Thanks for your help, I'll keep searching
Regards,
Warmy