Koru Club product sits between Silver status and Gold really. Like others mentioned, you only have access to the Koru lounge and no access with Star Alliance lounges (there are exceptions). You can also get discounted membership from your corporate employer or credit card. Some credit cards give you lounge passes for free after a certain spend

And lastly, Not sure if it is still allowed (no longer published on Air NZ website), if you don't fly often, just pay per entry into the Koru lounge.