Low iron levels are a symptom rather than a condition. If someone's levels on anything the blood test covers show up as outside the normal range, the medical will be referred, and the Medical Assessor has the power to require a letter from the applicant's own GP or a consultant to say that they have been investigated, and the reason for the symptom has been found to be (whatever), with (whatever prognosis) and (whatever) suggested treatment, if any. The point is to make sure that the unusual level is not due to something that will need expensive treatment for the NZ Health Service. In the case of your iron levels, it may well be that your doctor has already prescribed some treatment, or has decided that that level is normal
for you.
To shorten the process of the medical being referred, you can ask your doctor to write a letter about your iron levels that you send to INZ to catch up with your medical (giving all details of your application, and eMedical number). Medicals are scanned by computer and automatically referred, then there is a wait in a date-order queue to be looked at by a human being. With luck, by the time the MA looks at your form, there will also be your doctor's letter giving his/her opinion on your iron levels, which will answer all the questions that s/he would have wanted to have answered.