It is known that Police have already begun visiting people to do this, despite the fact that the amnesty on the newly banned rifles lasts until 30 September.  Why?

From the Council of Licensed Firearm Owners.

Council of Licenced Firearms Owners Inc.,

PO Box 24020, Manners Street, Wellington 6142 

http://www.colfo.org.nz

+64 27 442 3310

Search & Seizure — What You Should Know

Under the Search & Surveillance Act 2012 Police may search you, your home or vehicle, without a warrant, using force if necessary and seize any firearms and firearm licence that they find. They must have reasonable cause to suspect that you have firearms and are in breach of the Arms Act 1983, or due to your physical or mental state are unable to exercise proper control of your firearms, or may kill or injure any person, or a protection order has or may be taken out against you…