Air New Zealand Air Points - Earliest time to check in for a domestic flight?

I've booked a Grabaseat ticket AKL - WLG and don't have any luggage. As I can't choose my seat until I get to the airport, when is the earliest I can check in? (I hope to get a window seat.)
Any tips?
Thanks.

You can use the self check in as soon as the airport opens for the day.

Enjoy your flight..!!

Many thanks.^^