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 in standard bullion sizes - typically, one ounce of silver (abbreviated as 1 oz Ag, where "Ag" comes from the Latin word for silver, Argentum).
Note that the various mints have been authorised to use different effigies of the monarch (who, strictly speaking, is monarch of New Zealand and not of Niue), so the obverses are inconsistent.
The coins are "Non-Circulating Legal Tender" (NCLT) and not bullion because they are issued at prices much higher than their bullion value and are targeted at collectors who appreciate them for their artistic or sentimental value, and not at bullion investors. For similar coins issued at prices near the value of their precious metal content, see Silver Ounce (Bullion). | 
