If you are under 25 years old, you will be asked for your ID every time you buy alcohol or want to enter a pub, bar, club, etc. If you are sick of carrying your passport with you all of the time, you can apply for a Hospitality NZ 18+ card. It's an approved photographic Evidence of Age Document, like your photo driver licence or passport. It is a means for young people to establish that they are over 18 and entitled to purchase alcohol
To apply for your 18+ card you will need to be at least 18 years old, supply a passport-sized photograph, provide the number of a primary identification document (passport or NZ diver license), a proof of address (electricity or phone bill), and an "identifier" to fill out a section of the form.
Then you will have to fill out a form that you can pick up in any post office (click here to find your closest NZPost) or in some bottle shops, bars and clubs.
You can also download your application by clicking here.
Once filled, you need to take your completed application form to an NZPost outlet to get them checked and processed.
It takes 2-3 weeks to get it and the application fee is NZ$45.
While it is definitely not necessary, it's a quick process and very useful. When back home, the card is a great souvenir.
And by the way, it looks like this: