Hi,
basically nobody knows how long the registration process can take. Some people have made it in 8 weeks, but they didn't say how long it took them to prepare their portfolio, i.e. have everything ready for the board to be satisfied and accept the application.
Once you get NZ registration, you do not have to get an APC. You can also register as "non practising" and keep your name on the register for an annual fee.
The APC in NZ is valid from 1. April to 31. March each year.
See here:
The Physiotherapy Board of New Zealand > Annual Practising Certificate
The rest depends on how long you are going away for, but I wouldn't let registration with the HPC expire.
Cheers,
Fyzzio