Trying to Find Info on Mrs Catherine Nelson Who Married Walter Mcmillan Dunedin Nz 1868

This is what I know:

• Catherine Nelson married Walter McMillan in Dunedin, New Zealand 9/11/1868. On line it looks that they were married at the Knox Church, Dunedin, but I ordered the marriage certificate and it states that they were married at the house of Duncan McFarlane, Water St, Dunedin. There is no information on either parents, or where they were born.

• Catherine was born in 1841, but I can not find records of her being born in NZ.

• On her marriage certificate she is listed at Widow, so I am guessing that Nelson is her married name, and not her birth name.

• One of her daughters was Catherine McFarlane Mconie McMillan, on her certificate Catherine has listed her name as Catherine McFarlane McMillan (McFarlane and Mconie are not from the McMillan side).

• There was a Margaret McOnie who married Pollock on 5/5/1868 at Knox Church.

• The Toitu Otago Settlers Museum on their website have: Duncan McFarlane from Glasgow arrived 1849 on Mooltan (but I can't find the records) and I don't know if he is the person at the home of where they were married.

• I have found 4 Nelson men who died in Otago around the time she would have been old enough to marry prior to meeting Duncan McMillan: Cuthbert Nelson aged 12 1/6/1865, Frank Nelson, aged 24 25/8/1865, Henry Nelson (MD) aged 38 18/5/1867 and James Nelson not recorded 27/4/1865

• There appears that McFarlane is a common middle name of early Scottish settlers in the Otago area, but McOnie is uncommon.

Any help would be appreciated.
Amanda