... and also from Catholic baptisms (Christchurch Diocese) ... the following - which despite the year (1874), may be your Margaret Beatrice ?
ID Code : 12417
McGRATH - Margaret
Date of Baptism : 22 March* 1874
Parish Name : Lincoln
Parish ID : 30-----------------
* Noting that above date -
22 March - is the same as you've calculated as birth day and month, per Margaret ROGER's death cert.

[May be a coincidence of course, but still worthwhile purchasing copy of baptism certificate. The Parish of Lincoln is South of Christchurch, and quite a distance from Rangiora, but Peter McGrath is known to have been working at times, in or near to Lincoln. Have found further evidence to suggest that due to a lack of Priests, the Catholics of Christchurch took turns at worshipping in a variety of different parishes. ]
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This from the Index of Christchurch Catholic Baptisms :
"Computer-produced printouts of Baptisms are available at a cost of $5 (NZ$5).
Printout will normally include - Full name / date of birth / date of baptism / address / father and mother's names (often the mother's maiden name) / parish / godparents and celebrant's names. "Ordering Information :
Include ID Code and all pertinent info. as given on index.
Postal Address: Catholic Diocese of Christchurch / P O Box 4544 /Christchurch 8140, New Zealand.
[Not sure that they have a payment by credit card option ?? ]
Perhaps try an email firstly to the Diocesan Office ? (- they're most helpful).
http://www.chch.catholic.org.nz/?sid=1037---------------
more to follow ... >>
~ Lu