UK State Benefits:
When you emigrate [or return home] whatever benefits you have built up within the UK State system will remain in place until such time as you reach what would have been your normal UK retirement age – i.e. you do NOT lose these benefits.
When you reach that age your benefits can be paid in one of 3 ways:
NZ State Benefits:
As a new migrant you will be entitled to full NZ State pension once you have been here 10 years and worked at least 5 of those years since age 55.
This is a simplified version of the system but sums up your entitlement.
If you have any UK State benefits then these underpin the NZ benefits – e.g. if the NZ State benefits are $30,000 p.a. and the UK gives you [say] $20,000 then the NZ system will top you up to the $30,000 level!