It's definitely only for departures. However, in the past, they have let me in on arrival if I had extenuating circumstances. The first time, there was a bus strike in Wellington and I made it to the airport with only 2 minutes to spare, so had no time to eat or use the loo. When I got to Auckland, I went to the Koru lounge and explained, and they gladly let me in. The next time, I was with my preschooler, who had to use the toilet. Again, they were happy to let me in. I did ask nicely and knew that if they said no, I would have had nothing to complain about.

That said, I don't think Koru staff should ever be rude to passengers.