Yes -- and no. You have to remember that outside of a few countries, unlimited internet isn't ubiquitous. Admittedly, NZ is basically a worst-case scenario (we're connected to the rest of the world by a single pipe, access to which is - was - priced expensively), and we've recently seen an explosion of data caps ($130 for 550GiB from one provider, now), but that's only a very recent development. Some ISPs charge up to $2 a GiB - still.
So, using BT and seeding becomes a less valid option. Still, I'll happily admit that things are changing, and if NZ is a yardstick of the bad, then BT as a distribution mechanism (backed by, say, Akamai or other CDNs) for legal content could definitely take off.
Woulden't bittorrent be beneficial in NZ for popular torrents since not everybody needs to download the entire content through the Southern Cross Cable but instead could download some from local NZ peers?
In a perfect world: yes. In reality: your uploads count towards your data cap, and national traffic is no longer free (it used to be). When your data cap is expensive/small, seeding can cost you a significant amount of money.
Then there would have to be some feature that filtered non-NZ-peers. Would be great, but I don't think any bittorrent implementation supports this (yet).
That's local as in Local Area Network as in it discovers other peers inside your house. To discover other peers in the same ISP you'll need to wait a few more years for ALTO.
So, using BT and seeding becomes a less valid option. Still, I'll happily admit that things are changing, and if NZ is a yardstick of the bad, then BT as a distribution mechanism (backed by, say, Akamai or other CDNs) for legal content could definitely take off.