If you buy a vehicle with an expired licence label (vehicle licensing is sometimes incorrectly called 'registration'), you're required to pay licence fees from the date you acquire the vehicle. If the vehicle isn't going to be used on the road, you should apply for an exemption from licensing. (Do this after you have notified acquisition.) You'll need to fill out an Application for exemption from continuous licensing form (MR24), available online at the Transaction Centre or from an NZTA agent.
The seller is liable for any outstanding licence fees up to the date of the sale and acquisition.