Until the end of this month(June) Environment Canterbury are running a promotion where people can get a free Metrocard (while stocks last). There’s also a New World voucher giveaway.
Even if you have a Gold Card for off peak travel, having a Metrocard means that you would be able to access cheaper fares at other times.
It is also now possible to order a Metrocard online, and until the end of June the only cost of ordering a card this way is the postage.
Here is the link to the where you can get a Metrocard, and the online ordering form: http://www.metroinfo.co.nz/metrocard/Pages/WhereToBuy.aspx