This vehicle has been de-registered in the past and then registered again.
The registration may have expired because:
Passenger vehicles can only remain unlicensed for 12 months, or 24 months if currently owned by a vehicle trader. Non-passenger vehicles (trailers, ATVs, etc) can remain unlicensed for 24 months.
What you can do
Contact the seller and ask them about what happened.
If you still have doubts, you could have an independent mechanical inspection by a reputable company e.g. VTNZ or the AA.