Sorry, but I don't know how complete the MI transcripts are, nor whether there were gravestones with no inscription (but unlikely, unless worn off).
The burial records at the church should give the grave number so that it could be visited.

This is the 1851 census household transcription from Findmypast:
Name Relation Condition Se x Age BirthYear Occupation Disability Where Born:
CHRISTIAN, William Head Married M 72 1779 Tailor Ramsey Isle of Man
CHRISTIAN, Rebecca Wife Married F 48 1803 Ramsey Isle of Man
CHRISTIAN, William Son Unmarried M 17 1834 Grocers Ap Ramsey Isle of Man
CHRISTIAN, Jane Daughter Unmarried F 14 1837 Ramsey Isle of Man
CHRISTIAN, Edward Son M 12 1839 Ramsey Isle of Man
CHRISTIAN, John Son M 10 1841 Ramsey Isle of Man
CHRISTIAN, Ann Daughter F 7 1844 Ramsey Isle of Man
--------------------------------------------------------------------------------
RG number:HO107 Piece:2524 Folio:376 Page:7
Civil Parish:Maughold
Address:
Maughold Street, Ramsey Isle of Man

[Actually they wrote "Sailor" but it looked like Tailor to me on the scan, so I checked 1841 where he was definitely Tailor.]

In the 1841 census John's age is given as 1 month. He was bapt on 8 June 1841 Ramsey. The census was taken on the night of March 30/31st, so his birthdate end-Feb?

I didn't see him in any records with a second name.

What date did he go to NZ?

Sue