Just had a response from my ISA provider (Virgin) concerning my shares ISA (growth). Thought I'd repeat it for all those with ISA's - however don't know if this is a government rule, or just what Virgin do.
" We are writing to confirm that in order to contribute into an ISA a customer must be originally resident in the UK. This means that a customer must have been resident in the UK for 184 days or more in the current tax year. Exceptions to this rule applies to members of the armed forces and crown employees.
You can continue to invest into your ISA up until the day you move outside the UK, or if you have been resident in the UK for 184 days or more you can continue to invest into your ISA for the current tax year only.
We can also confirm that the exisiting funds in your ISA can remain as tax free investments regardless of your residency status"
I found this refreshingly clear financial information for once. But as Shaker Mike points out, if you do keep your investment, you might be liable for NZ tax. Haven't quite worked out that side of things yet - somthing about 2 years to sort it out, but might be liable for foreign investments tax. Still I'm getting there.