It has to be a panel doctor
This is the part that costs people the most time and money, because it is easy to get wrong before you know the rule exists.
Immigration New Zealand only accepts medical examinations and x-rays from doctors and radiologists on its approved panel. A full medical from your own GP, however comprehensive and however recent, is not accepted and cannot be converted into an accepted one. The money is simply gone.
The panel is published by country. Check it, book with someone on it, and confirm they are still listed when you book rather than relying on a recommendation from someone who used them two years ago.
Who needs one
It depends on the visa you are applying for and how long you intend to stay. Longer stays and residence applications generally require more than short visits, and there are age thresholds for some parts of it.
The requirement also depends on where you have lived, because tuberculosis screening is targeted at people who have spent time in higher-incidence countries. That is why two people applying for the same visa can face different requirements.
Immigration New Zealand’s page on who needs an x-ray or medical examination is the one to work from, and it is worth checking rather than assuming, in either direction.
What is actually being assessed
Two questions, and they are separate.
Are you likely to be a danger to public health? This is mainly about active tuberculosis, which is why the chest x-ray is so central to the process.
Are you likely to impose significant costs or demands on New Zealand’s health or special education services? This is the broader test, and it is the one that surprises people. It is not about whether you are unwell today. It is about the projected cost of a condition over time.
Conditions that people manage perfectly well in daily life can raise this question, because the assessment is about cost to the system rather than about your capability. That is uncomfortable, and it is how the rule works.
If something is found
An unexpected finding is not automatically the end of the application. There are processes for it, including a medical waiver in some circumstances, where other factors in your case are weighed against the health cost.
What matters is how it is handled. A condition that is disclosed, documented by your treating specialist, and accompanied by evidence about prognosis and cost is in a far better position than one that emerges from the examination after you said nothing.
This is one of the clearest cases for getting licensed advice before you submit rather than after a letter arrives, because the framing of a waiver request is most of its strength.
Results expire, so time it
Like police certificates, medical results are valid for a limited period. Too early and they can expire during processing. Too late and the application waits.
Medicals and police certificates are the two documents that come from third parties on their own timetable, and between them they account for most of the delay in otherwise complete applications. Start both at the beginning, and assemble the rest of the application while they are in progress.