Once they get residency they will be able to make an appointment with WINZ Superannuation Dept (not sure if they will need to complete some time living here or not but WINZ super will answer any questions you have just give them a call) complete forms and it will take approx. three months to organise the payment of the UK pension into the NZ government on a monthly basis, basically they will authorise on the paperwork that they except a bank account being opened with Westpac where the payment will be made into but this is a non accessible account to your inlaws. NZ super will be paid thereafter until they pass and then it will pass with them but you will need to notify the UK pensions complete with certificates so that it doesn't keep being paid into the NZ coffers. The three months will need to be done on a conversion via HiFx or similar on a monthly basis until they commence NZ super payments. I am not sure about taxation this end I cant remember if super is taxable or not.
One other thing whoever passes first the remaining spouse can claim benefits from the UK for a 12 month period thereafter after filling in the usual forms and submitting to the UK pensions dept.
The reason I know this is because it was what I had to do with regard to my hubbies state pension.
If they have occupational pensions then that's another matter altogether and will be taxable by both the UK and NZ at the end of the 4 year tax free period allowed for immigrants. Hope this helps a little. Cheers Anni