Student loan borrowers have an obligation to notify the IRD of their contact details. If you do not do so, the fact that the IRD sent you letters but you didn't receive them because they didn't have your correct address won't save you from arrest.
But what about the many student loan borrowers who do tell the IRD where they have moved to, but the IRD fails to update their records? You can bet that if it is your word against the IRD, you will lose. This means that if you move, and you tell the IRD that you have moved, but you don't know that there is a problem with your student loan account because the IRD sent your mail to the wrong address, that you will be at risk of arrest and all the consequences that will flow from that. It will now be very important that you keep written records of all communications with the IRD and that you have written acknowledgement from them of any important communications, like a change in your address. One way that you can do this is by signing up for an online account with the IRD. When you change your address, advise them through secure mail and make sure you print and keep the receipt. This way if the IRD's mistake exposes you to arrest, you will have proof that should help you.