The history of this island is quite fascinating. Along the years it served several purposes to the inhabitants of the country: it used to be a signaling post for ships, a farm land and an army defense post during the war. It wasn't until 1970 that the island became a reserve and its restoration process started. You can learn more about the history of Tiritiri on its official website.
Nowadays the island is managed by a huge group of volunteers and the Department of Conservation. Volunteers offer guided walks that you should definitely do when you visit. They'll help you understand how this amazing place became a reality plus they'll give you tips about the flora and fauna of the place.
You can get to Tiritiri via ferry from Auckland or from Gulf Harbour. The Department of Conservation offers basic accommodation for visitors who wish to spend more than one day in the island which I also recommend. Night is the perfect time to spot some birds such as Little Spotted Kiwi or Little Blue penguin. During our visit we were fortunate enough to see both of these species, which was a great experience.
If you happen to live or visit Auckland this is an absolutely must-do, you will not regret it :)