Haven't found anything that hints at a likely age for this John KILLEY

[I'm guessing that as he was self-employed in what seems to be a job requiring some skill (well-sinking), then maybe he was in his mid-30's in 1857 ? Weren't wells lined with bricks ? Possibly he was the "bricklayer" chap in 1851-52 ??]
Aah ... but did eventually find the name of his wife.

Mary Ann KILLEY.
John KILLEY had published, some "notices of caution" asking that his "runaway" wife not be given credit, etc. etc. (Poor lady).
"Daily Southern Cross"
-- 24 October 1854 - page 4/ column 1
-- 6 February 1857 - page 2 / column 1
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Not sure if any of this is going to be of help to you ? A number of websites carry "a list of persons hanged in NZ" but all have the same sketchy detail for KILLEY.
Will keep on looking though ...
~ Lu