well they could be anywhere tho suspect Australia/NZ extremely unlikely.

Seriously post 1815 there were fewer wills/administrations for those with no or little estate to leave, their last child was I think born 1803 (if their children emigrated then a reasonable chance that aged parents moved to be with one of their children) - they are most probably dead by 1841 (+ almost certainly by 1851) so no easy way via census returns to find them if they merely moved to the adjacent Island - if they owned property then there should be a paper trail for this. Otherwise I suspect it will be pure luck that one or both are mentioned in some otherwise unrelated document