iCloud+ plans and pricing - Apple Support (NZ) shows you that the three plans cost in NZ$
50GB: $1.69
200GB: $4.99
2TB: $16.99
and the instructions for purchasing a subscription are linked at the top of the page:
You can upgrade to iCloud+ from your iPhone, iPad, iPod touch, Mac, or PC. Examples of how to do this on each of these devices are in the document.
As you seem to have a Windows computer -
How to upgrade on your Windows PC
- Open iCloud for Windows.
- Click Storage.
- Click Change Storage Plan.
- Choose a plan, then click Next.
- Enter your Apple ID password, then click Buy.
Check first the System requirements for iCloud.
PC
- Microsoft Windows 10*
- iCloud for Windows 11 or later
- iTunes 12.7
- Outlook 2016 or later
- Firefox 45 or later, Google Chrome 54 or later (desktop mode only), Microsoft Edge, or Opera
* Windows 10 May 2019 Update or later is recommended for iCloud for Windows 10 or later.