Hey,
Just a bit of information for people. I am a UK qualified physio that gained NZ rego in the last year and am now in Australia. I have full AHPRA rego as a result of the trans tasman agreement.
The trans tasman agreement is a legal agreement between the governments of Australia and NZ. The agreement basically stipulates that anyone who is registered for a profession in NZ (not just physio) can practice that profession in Australia provided that the profession is sufficiently similar in it's role and expectations. Therefore if you are registered for physio in one of the countries you can be registered in the other.
However remember that when you send in the application for NZ you are signing a declaration that you intend to work there. And even though it may be cheaper it should not be viewed as the easier option as the NZ board rigorously assess applications and will continually request any information they require. If they are still unsure of your competency they may request that you take practical exams in NZ. I have been to both countries on a working holiday and it made no sense to go through the Aussie reg process considering the cost and length of time. And if I had gained limited registration then it wouldn't have transferred to NZ as general registration.
As far as I am aware you only have to re-register with AHPRA, it is not a requirement to re-register in NZ every year.