If you fly predominantly on the Air NZ network then I think koru is fine for priority checkin baggage and seat select. You have lounge access in a bunch of international destinations as well. You know you can get the discounted koru membership via the credit card right? The nice thing is you don't have to count or worry about SP earn.
If you want benefits flying Star Alliance partners of NZ also then you need Gold.
If you are unsure then you need to weigh up the cost of Koru vs the cost of attaining gold (even if that means a status run ie fly for the SP earn). Then you can see if it's worth it to you. You can play around with the Air NZ calculator as well or dummy bookings to see how much you'll earn and where you can go. Also take into account whether you can maintain gold the following year. The retention levels are lower.
Make sure you're making the most of your NZ credit cards for SP earn.
You haven't said how far away from gold you are. How many SP?