Hello again @Korina ... I contacted the New Zealand MetService and heard back! They stated they are aware their weather data access is more restricted than other countries, so they are in the process of making changes. Hooray!
Apparently they are developing an API that will make a wider range of data publicly available, specifically aimed at supporting the development of innovative weather products, It should be available later this year. Current publicly available data information can be found here: https://about.metservice.com/our-company/about-this-site/open-access-data/
From what I can see though, radar data is already now available. Although not sure if that is the data that will work with Windy right now or you need to wait for their new API? Either way, hopefully this gets us closer to getting NZ radar data back on Windy! :)