Niue, a sovereign state in free association with New Zealand, uses two official legal tender currencies. The New Zealand Dollar is the circulation currency for daily transactions, while the government also authorises legal tender coins in the Niue Dollar currency for collector's purposes.
A number of mints issue a large variety of commemorative, bullion and collector coins under the authority of Niue. These coins are dedicated to historical or general popular culture themes not related to Niue itself. Many of them are made using "smartminting" techniques and have features not generally found in circulation coins - e.g. high relief, full colour printing and/or different forms, including fully three dimensional (3D).
Some coins are marketed in standard bullion sizes, like half ounce silver, one ounce silver etc, and some of those are also denominated as $1, for which see respective lists; this particular coin denominated as One Niue Dollar is not in a standard ounce-based size.
This rectangular "pendant" coin is part of a Mint of Poland series featuring the Chinese Lunar Calendar, and is dedicated to the 2016 Year of the Monkey.
The mint says about it:
According to the Chinese calendar, the Year of the Monkey begins on 8 February 2016 and ends on 27 January 2017. Monkey, the ninth animal of the Chinese zodiac, is a sign of a talented inventor and a skillful leader. Therefore, it is not surprising that people born in the year of the Monkey are keen to compete and take up new challenges. Quick-witted and flexible, they are also outgoing and proactive, always eager to make new friends and pursue their goals. | 
Search for Niue: Silver Ten Grams 2016 Year of the Monkey - Pendant on eBay 